US Vice President Kamala Harris is set to begin her trip to Poland and Romania tomorrow as the Biden administration continues to show support for Ukraine and US eastern NATO allies as the he Russian invasion of Ukraine is entering a dangerous new phase.

Harris’ trip will take place from Wednesday through March 11 and will include stops in Warsaw and Bucharest, according to a Friday announcement from the White House.

She will meet with leaders from both countries to coordinate their response to the Russian invasion and discuss how the United States can further support countries neighboring Ukraine as they prepare to welcome Ukrainian refugees fleeing the conflict. .

The White House said the leaders “will also discuss their continued support for the people of Ukraine through security, economic and humanitarian assistance and our determination to impose severe economic consequences on Russia and those who are complicit in the Russian invasion.

The trip comes as the Pentagon on Tuesday rejected Poland’s offer to transfer its MiG-29 fighter jets to the United States for delivery to Ukraine.

Pentagon Press Secretary John Kirby said in a statement that the Pentagon does not believe Poland’s proposal is ‘tenable’, just hours after Polish officials issued a statement saying the government was ready. to deploy all of its MiG-29 fighter jets on the US Air Force’s Rammstein. Air Base in Germany so that they can then be supplied to Ukraine in its fight against Russia.

Kirby said the decision to transfer Polish-owned planes to Ukraine was “ultimately a decision for the Polish government”, adding that the proposal shows the complexities of the issue as Russia has made threatening statements about arms supplied to Ukrainians for use against Russia. forces.

The idea as laid out by Poland was too risky, Kirby said, as the United States and NATO seek to avoid an outright conflict between the alliance and Russia.

More background: The Defense Department statement was released late Tuesday after the Polish proposal caught the Biden administration completely off guard, multiple sources told CNN.

While Ukrainian President Volodymyr Zelensky pleaded for more planes amid the Russian invasion, the offer had not been discussed with the United States before making it public and Polish officials have not. Nor spoke with Secretary of State Antony Blinken when he was recently in Poland.

US officials have privately weighed sending planes to Ukraine, but have repeatedly noted the difficult logistical challenges associated with the operation.

Poland’s surprise announcement complicates what had already been a high-stakes visit by Vice President Kamala Harris, who is due to land in Warsaw on Wednesday evening.

Harris was expected to discuss the fighter jet issue while in Poland, officials said. The White House previously said it was in talks with the Polish government about a plan for Poland to supply Ukraine with its Soviet-era fighter jets and for the United States to fill the planes with F-16s.

Harris is still scheduled to leave Wednesday morning for Poland, but there are now intensive conversations within the administration about how to work with Poland to reach some sort of deal to allow planes to reach Ukraine.

CNN’s Jeremy Herb, Kylie Atwood, Jennifer Hansler and Oren Liebermann contributed to this post.