CHISINAU, Moldova—Secretary of State Antony Blinken said the U.S. government is actively pursuing ways to address Ukraine’s request for combat jets and to replenish Poland’s arsenal should it hand over Soviet-era planes to its besieged neighbor.

Ukrainian President Volodymyr Zelensky on Saturday made an impassioned plea to Capitol Hill for assistance in obtaining more lethal military aid, especially Russian-made jet fighters that Ukrainian pilots can fly. Ukraine’s military has largely relied on surface-to-air missiles to...

CHISINAU, Moldova—Secretary of State Antony Blinken said the U.S. government is actively pursuing ways to address Ukraine’s request for combat jets and to replenish Poland’s arsenal should it hand over Soviet-era planes to its besieged neighbor.

Ukrainian President Volodymyr Zelensky on Saturday made an impassioned plea to Capitol Hill for assistance in obtaining more lethal military aid, especially Russian-made jet fighters that Ukrainian pilots can fly. Ukraine’s military has largely relied on surface-to-air missiles to challenge Russian military planes flying over the country, with some apparent success.

“We are looking actively now at the question of airplanes that Poland may provide to Ukraine and looking at how we might be able to backfill should Poland choose to supply those planes,” Mr. Blinken said on Sunday.

Mr. Blinken said the U.S. is working with Mr. Zelensky and other Ukrainian officials to get an “up-to-the-minute assessment of their needs.” The U.S. and partners will then assess what can be provided, he said a day after meeting Ukraine’s Foreign Minister Dmytro Kuleba during a brief gathering at the Polish-Ukrainian border at which Mr. Kuleba asked the U.S. to do more to aid its country militarily.

Polish air force MiG-29 jets on Armed Forces Day in Warsaw in 2017.

Photo: leszek szymanski/European Pressphoto Agency

“I can’t speak to a timeline, but I can just tell you that we’re looking at it very, very actively,” Mr. Blinken told reporters during a stop in Moldova as he traveled across Europe to reassure allies there about U.S. support in the wake of Russia’s invasion of neighboring Ukraine.

Ukrainian officials are pushing Western governments for a swift provision of combat jets—or, alternatively, a no-fly zone—saying either would help save lives at risk as Russia continues to bombard cities in the nearly two-week-old war. “If we keep this decision process going, more casualties will happen,” a Ukrainian government official said.

Yet European governments worry that donating jet fighters at this stage risks retaliation from Russia, potentially dragging Poland—and with it, the North Atlantic Treaty Organization alliance—into the war.

On Sunday, Poland’s prime minister’s office dismissed reports of a potential arrangement for the country to hand over combat planes to Ukraine. “Poland won’t send its jet fighters to #Ukraine as well as allow to use its airports,” the chancellery of Prime Minister Mateusz Morawiecki tweeted. “We significantly help in many other areas.”

Two Polish officials close to the issue said Poland would at least consider the proposal Washington was willing to put together. The Polish air force inventory still includes some Soviet-made MiG-29 combat jets, similar to those Ukraine has had in its fleet. Poland operates 21 single-seat and six two-seat MiG-29s, according to Jane’s World Air Forces.

“We understand that Poland needs to be protected, but it is a win-win game. We will have new jets and Poland will have new jets,“ said the Ukrainian official, adding talks were ongoing.

Both Ukraine and Poland would prefer for the decision to be agreed on collectively by NATO states, senior officials from both countries said.

“It’s possible. It’s too early to say,” said a senior Polish official. “We need to act as one, as Western countries, as NATO, an alliance.”

A Polish government spokesman didn’t return requests for comment.

“We have to do it,” said Latvian Defense Minister Artis Pabriks, adding, “We must assist this country to defend itself.” The decision, he said, could have been taken earlier. He also pushed back on the idea Russia would see it as a reason to widen the conflict. “If Moscow or the Kremlin would like to fight a war against NATO or Europe, they could always find a reason,” he said.

NATO officials have said they wouldn’t put planes over Ukraine to enforce a no-fly zone. Establishing such a no-fly zone, Russian President Vladimir Putin has said, would be tantamount to becoming a party to the conflict.

Mr. Zelensky made his request for jets in a call with more than 200 House and Senate members, said people who participated. He spoke for about 25 minutes before taking questions.

When Senate Minority Leader Mitch McConnell (R., Ky.) asked Mr. Zelensky what one thing he needed most, the Ukrainian president replied with the need for jet fighters, according to two people on the call.

Any deal would likely require White House approval and congressional action, U.S. officials said.

Poland isn’t the only Central European country still flying the twin-engine MiG-29 jet, though its planes are among the most modern. Ukrainian officials are also hoping Slovakia will reach a similar agreement.

A senior Slovak official said the government wasn’t considering that option.

The debate around resupplying Ukraine with combat planes, to battle Russian troops that have been bombing and shelling Ukrainian cities, has now raged for more than a week with little progress.

One underlying tension is whether arranging the transfer of the jets to Ukraine would amount to a more direct involvement in the conflict. Some lawmakers say it wouldn’t. The plea for the planes, as well as drones, has prompted lawmakers from both parties to urge the Biden administration to do more to facilitate the transfer of the Russian jets.

—Gordon Lubold and Michael R. Gordon contributed to this article.

Write to William Mauldin at william.mauldin@wsj.com, Drew Hinshaw at drew.hinshaw@wsj.com and Nancy A. Youssef at nancy.youssef@wsj.com