The US Department of Defense has described a proposal by Poland to hand over MiG-29 fighter jets to Ukraine with an intermediate stop at a base in Germany as "unsustainable".

The proposal poses "difficult logistical challenges" and there are "serious concerns" about the geopolitical dimension, Pentagon spokesman John Kirby said on Tuesday evening.

A senior diplomat at the State Department, Victoria Nuland, described Poland's offer, which had apparently not previously been coordinated with the government in Washington, as a "surprising move" in a Senate hearing.

The Polish Foreign Ministry said on Tuesday evening that the government was ready to move all MiG-29 fighter jets to the American Ramstein Air Force Base in Rhineland-Palatinate immediately and free of charge and to make the machines available to the United States.

At the same time, the United States is being asked to provide the country with used aircraft with the appropriate operational capability, it said.

"Serious concerns for the entire NATO alliance"

Kirby said the Pentagon is in contact with Warsaw following the Polish statement.

The decision to let Ukraine have Polish fighter jets is ultimately a matter for the Polish government.

But the proposal underscores "the complexity of this issue," Kirby said.

The idea that fighter jets handed over to the US military were flying into contested Ukrainian airspace from a US or NATO base in Germany during the war with Russia raises "serious concerns for the entire NATO alliance," Kirby said further.

"It's just not clear to us that there's a valid justification for it."

Kirby added in the unusually clear statement that the United States would continue to discuss the issue with Warsaw and NATO allies.

The United States, Germany and numerous NATO allies are supporting the government in Kyiv by supplying arms and equipment to the Ukrainian military.

Warsaw has repeatedly ruled out direct delivery of planes to the neighboring country – which Russia could interpret as direct intervention in the war.

"Decisions on the delivery of offensive weapons must be made unanimously at the level of the entire NATO," said Prime Minister Mateusz Morawiecki in Oslo on Tuesday evening.

Poland cannot take any independent steps because it is not involved in this war.

Foreign Minister Annalena Baerbock (Greens) was reluctant to comment on the considerations on Tuesday.

One must ensure that this war does not spread to NATO territory, said Baerbock in "Bild-TV".

Even with weapons deliveries, no steep template should be given for saying "we are taking part in the war," said Baerbock.

“Quite complicated logistics”

According to reports, Kyiv asked for the planes because the pilots of the Ukrainian Air Force are trained to fly the models initially developed in the former Soviet Union.

A possible release of the combat aircraft has also been discussed with the American government for days.

The White House always emphasized that it was a decision by Poland as a sovereign state.

"We have in no way opposed Poland transferring planes to Ukraine," President Joe Biden's spokeswoman Jen Psaki said Monday.

However, “a number of challenging practical questions are associated with the project.

"Where are you going to take off from?

Where will they land?

These are all very important questions here,” Psaki said.

The “rather complicated logistics” of supporting Poland with other fighter jets after a possible transfer to Ukraine are being examined, she said.

Russia warned on Sunday that it would be considered interference in an armed conflict if states allowed Ukrainian planes to land on their airfields, which then attacked Russian forces.

American Vice President Kamala Harris is scheduled to visit Poland and Romania this Wednesday to show the "strength and unity" of NATO and American support for the Eastern European partner states in the face of the Russian war of aggression, as the White House said last week .