Ukrainian President Volodymyr Zelensky on Monday launched a fresh appeal for military aid as the country prepared for some of the conflict’s heaviest fighting so far, while France sent a police unit from its armed forces to Ukraine to investigate possible war crimes, the first disclosed deployment of military personnel in Ukraine from a North Atlantic Treaty Organization country.

Meanwhile, Ukrainian authorities said they were investigating a claim by its troops in the besieged city of Mariupol that they came under a Russian chemical weapons attack Monday. The Russian government hasn’t commented publicly about the alleged use of chemical weapons in Mariupol.

In a virtual address to South Korean lawmakers Monday, Mr. Zelensky pressed Seoul to deliver more than the humanitarian assistance and nonlethal aid it has given so far, stating a need for heavier equipment to fight Russian tanks and missiles. “We thank South Korea for the help you have provided, but to survive from the war with Russia we need more help,” Mr. Zelensky said.

Ukraine has stepped up its pleas for heavy military equipment to fight the conventional tank and artillery battles that are expected in the Donbas area of eastern Ukraine. Having failed to capture Kyiv, Russia has shifted its objective to seizing the parts of the eastern Donbas area it doesn’t already control. Both Ukraine and Russia are moving troops and equipment into the region, with Ukraine redeploying combat units from northern areas that it recovered.

Ukrainian President Volodymyr Zelensky addressed South Korean lawmakers on Monday.

Photo: chung sung-jun/pool/Shutterstock

Workers loaded a destroyed Russian tank onto a transport vehicle Monday in the village of Andriyivka, near the Ukrainian capital Kyiv.

Photo: Efrem Lukatsky/Associated Press

Railway connections were disrupted overnight in the Russian town of Shebekino, near the Ukrainian border northeast of Kharkiv, the regional governor told the RIA news agency. There were no casualties, he said, and the cause of the disruption was being investigated.

Russia uses the railways heavily to reinforce and resupply its troops preparing the Donbas offensive, and Shebekino, in Russia’s Belgorod region, sits on one of its main rail connections to the area. Russia recently said two Ukrainian helicopters launched an airstrike on a fuel depot in Belgorod, a claim Kyiv didn’t confirm or deny.

Skirmishes along the contact line in Donbas and nearby regions occur daily, and Russia has continued its long-range missile campaign on Ukrainian infrastructure. The Russian Defense Ministry said Monday that missiles destroyed a weapons and military equipment repair base of the Ukrainian air-defense forces overnight, as well as two ammunition depots. Those claims couldn’t be independently verified.

The Ukrainian unit deployed in Mariupol, the Azov regiment, said Russian forces dropped an unknown chemical substance from a drone, causing respiratory and nervous-system symptoms among its defenders and civilians.

No independent evidence of the attack emerged Monday from Mariupol. Oleksiy Arestovych,

a senior adviser to the Ukrainian president, said in a social-media post that the government is “checking the information about a possible chemical attack against the defenders of Mariupol.”

The Azov regiment, a unit of the Ukrainian military formed from a far-right volunteer force that was created in 2014, has been resisting Russia’s military in Mariupol alongside Ukrainian marines and other forces for more than a month, despite being surrounded and cut off from resupply.

Azov’s commander Andriy Biletskiy said in a video recording that the chemical substance affected three people in the Azovstal industrial area of Mariupol, his unit’s stronghold, but didn’t cause “catastrophic consequences.”

Western officials have warned for weeks that Russia could use chemical weapons in Ukraine, and Russian forces were supporting the regime of Syrian President Bashar al-Assad when its military was credibly accused of using chemical weapons.

Just hours before Monday’s alleged attack,

Eduard Basurin, military spokesman for the pro-Russian statelet in Donetsk, was quoted by Russia’s RIA state news agency as calling for the use of “chemical troops, which will find a way how to smoke the moles out” in Mariupol.

In his nightly address, Mr. Zelensky said the government is preparing “for a new stage of terror against Ukraine,’’ noting Mr. Basurin’s comments. “We take this as seriously as possible,’’ Mr. Zelensky said.

Mr. Basurin’s Donetsk People’s Republic, formed in 2014 and recognized by Moscow as an independent state in February, claims Mariupol as part of its territory and has sent its own fighters there alongside regular Russian troops.

Ukrainian authorities say as many as 10,000 people died in Mariupol in weeks of fighting that leveled much of the city. Some 400,000 people lived in Mariupol before the war.

Officials in the U.S. and U.K. said they were aware of the reports of a potential chemical munition used in Mariupol, but hadn’t confirmed their authenticity.

“These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia’s potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine,” Pentagon press secretary John Kirby said in a statement.

Ukrainian and Western authorities separately pushed ahead with investigations into alleged war crimes committed in formerly Russian-held towns in northern Ukraine before Moscow’s withdrawal at the end of March. The French government said its gendarmes, a police force that is part of the French army, would assist local authorities in probing any war crimes around the capital, Kyiv.

French prosecutors said last week they were probing whether war crimes had been committed in Ukraine following accounts of rape and killings of hundreds of civilians in Bucha and other formerly Russian-occupied towns.

Prosecutors also opened a separate probe March 16 following the killing two days earlier of a Franco-Irish journalist and his research consultant near Kyiv.

Russia has denied targeting civilians in its military assault on Ukraine and called the video and photos from Bucha staged.

In Moscow, Austrian Chancellor Karl Nehammer met with Russian President Vladimir Putin, in what was the first meeting between a European leader and Mr. Putin since the start of Russia’s invasion.

Mr. Nehammer, who also met Saturday in Kyiv with Mr. Zelensky, asked the Russian leader for a cease-fire, for safe passage for civilians encircled by Russian forces and for supplies to occupied regions, the chancellor said in a statement. Ukrainian authorities have said Russia hasn’t allowed civilians to flee along evacuation corridors toward central Ukraine.

He said he warned Mr. Putin that continuing the invasion would incur more sanctions and that those responsible for war crimes should be brought to justice.

“This is not a friendly visit,” Mr. Nehammer said. “I have just returned from Ukraine and I have seen with my own eyes the immeasurable suffering caused by the Russian war of aggression.”

Austria, which is heavily dependent on Russian natural gas, has been blocking sanctions against Russian energy imports to Europe together with Germany and other nations.

European Union foreign ministers discussed the bloc’s efforts to pressure Russia’s economy with sanctions at a meeting Monday, including the possibility of further energy sanctions. “Nothing is off the table, including sanctions on oil and gas,” EU foreign-policy chief Josep Borrell said after the meeting, which was held in Luxembourg. “But today no decision was taken.”

The EU approved a fifth package of sanctions Friday that included the first significant ban on imports of Russian energy—coal. However, the bloc remains deeply divided over whether to advance with further energy import bans, starting with oil.

A man’s body was removed Sunday from a mass grave in Bucha, near Kyiv, to be identified at a morgue.

Photo: Rodrigo Abd/Associated Press

Bunk beds filled a hall used as a shelter for displaced people inside the Arena Lviv stadium in western Ukraine on Monday.

Photo: Miguel Gutierrez/Zuma Press

A senior EU official briefed on Monday’s discussions said there was little detailed discussion of the options Monday and no progress in narrowing the divide within the bloc. A group of countries, led by Germany, oppose a speedy cutoff of oil imports. Others, led by Poland, are urging the bloc to stop buying Russian energy.

U.S. officials say they have urged the Indian government not to increase imports of Russian energy. President Biden met virtually Monday with Indian Prime Minister Narendra Modi as Washington presses New Delhi to take a tougher stance against Russia. India has avoided publicly denouncing Moscow. Mr. Modi said during the meeting that he has spoken several times with both Mr. Putin and Mr. Zelensky and advised the two leaders to engage in direct talks.

Western and Ukrainian officials say the timing of Russia’s next major campaign is up to Moscow, which may press the eastern offensive imminently with available forces, or wait a few weeks to reconstitute units that suffered losses in northern Ukraine.

The Russian Defense Ministry said that its missiles targeted a Ukrainian repair base near Velyka Novosilka in the Donetsk region, and that an ammunition depot it targeted near Mykolaiv, in southern Ukraine, was also destroyed.

Drone imagery shows the remains of a theater destroyed during the Russian assault on Mariupol last month.

Photo: STAFF/REUTERS

A family waited Monday in a temporary shelter at Lviv’s central train station before departing to Poland as refugees.

Photo: Joe Raedle/Getty Images

More Western companies announced sales or suspensions of their Russian operations Monday following sanctions imposed by the European Union and the U.S. Further, French bank Société Générale SA said it would cease its banking and insurance activities in Russia, including selling Rosbank, while Swedish telecommunications company Ericsson AB said that it was suspending its business in Russia indefinitely.

Société Générale said it was selling its entire stake in Rosbank and its Russian insurance units to Interros, a conglomerate controlled by metals billionaire Vladimir Potanin. Mr. Potanin and Interros have eluded sanctions from the U.S., EU and the United Kingdom. He was included on Canada’s list of sanctioned individuals last week. Société Générale has had investment banking operations in Canada since 1974.

—Bojan Pancevski, Yuka Hayashi and Patricia Kowsmann contributed to this article.

Corrections & Amplifications
Ukrainian President Volodymyr Zelensky met with Austrian Chancellor Karl Nehammer on Saturday. An earlier version of this article incorrectly said Mr. Nehamer’s planned Monday meeting with Russian President Vladimir Putin came a day after his meeting with Mr. Zelenksy. (Corrected on April 11.)

Write to Matthew Dalton at Matthew.Dalton@wsj.com, Yaroslav Trofimov at yaroslav.trofimov@wsj.com and Mauro Orru at mauro.orru@wsj.com