US Vice President Kamala Harris will travel to Poland on Wednesday to discuss how to provide "military assistance" for Ukraine, White House officials said, hours after Washington rejected Warsaw's offer of Soviet-era fighter jets.

The trip was planned before Poland took the United States by surprise on Tuesday by offering to send its Mig-29 jets to Ukraine via a US air base.

Washington rejected the proposal, with Pentagon spokesman John Kirby saying the prospect of the jets flying from a US-NATO base "into airspace that is contested with Russia over Ukraine raises serious concerns for the entire NATO alliance."

Harris will meet with Polish President Andrzej Duda and Prime Minister Mateusz Morawiecki on Thursday, according to US officials who requested anonymity.

When asked about the fighter jets, a White House official did not address the subject directly.

"We have been in dialogue with the Poles for some time about how best to provide a variety of security assistance to Ukraine. And that's a dialogue that absolutely will continue up to and as part of the vice president's trip," she said.

Canadian Prime Minister Justin Trudeau, who is visiting Poland, is also expected to meet Harris.

The vice president's schedule includes meetings with refugees who have fled the war and staff from the US embassy in Kyiv who have left the Ukrainian capital.

She will also head to Romania on Friday to meet with President Klaus Iohannis.

This is the third trip to Europe for the vice president, who has met President Emmanuel Macron in France and attended the Munich Security Conference.