The Pentagon on Tuesday rejected an offer from the Polish government to send its MiG-29 fighter planes to a United States air base in Germany for eventual use by Ukraine, a rare note of disunity between two NATO allies as they confront Russia.

The disagreement underscored the pressures the United States and its allies are under as they seek to provide military aid to Ukraine in its fight against Russia without getting pulled into a wider war.

Ukraine has been pleading for more warplanes, and American officials have raised the possibility that Poland could supply Ukraine with its older Soviet-era fighters in return for U.S. F-16s to make up for the loss. Ukrainian pilots are trained on the Russian aircraft.

Poland’s minister of foreign affairs said in a statement earlier on Tuesday that the country was ready to deploy its MiG-29 jets to the Ramstein Air Base in Germany, where they would be placed at the disposal of the U.S. government. In return, Poland expected the U.S. to provide it with used aircraft of comparable capabilities, the statement said.`

But a Pentagon spokesman, John F. Kirby, said Poland’s proposal to send the planes to a U.S. base in Germany, which caught American diplomats by surprise, was not workable. In a statement, he said the prospect of fighter jets departing from a U.S.-NATO base in Germany and flying into “airspace that is contested with Russia over Ukraine raises serious concerns for the entire NATO alliance.”

“We will continue to consult with Poland and our other NATO allies about this issue and the difficult logistical challenges it presents, but we do not believe Poland’s proposal is a tenable one,” he said.

The Polish prime minister, Mateusz Morawiecki, speaking at a news conference in Oslo on Tuesday, said that while his country was prepared to hand its fleet of jet fighters over to the United States military at Ramstein, Poland will not act unilaterally to give the warplanes directly to Ukraine.  

“Any decisions on delivering offensive weapons have to be taken by the entire NATO and on a unanimous basis,” he said. He added that Poland was not a party to the war.

Ukraine’s president, Volodomyr Zelensky, has pleaded with the NATO allies to establish a no-fly zone over the country, but so far the alliance has rejected the proposal because it would almost certainly lead to a wider war between the allies and Russia.

The NATO secretary general, Jens Stoltenberg, said on Friday that a no-fly zone could cause a “full-fledged war in Europe involving many more countries and causing much more human suffering.”

On Sunday, the Russian Defense Ministry had warned neighboring countries against holding Ukrainian military aircraft, saying that it “could be considered as those countries’ engagement in the military conflict.”

Poland has become a critical piece in NATO’s efforts to help Ukraine and contain Russia. More than 10,000 American soldiers are now stationed there as part Washington’s attempt to shore up the alliance’s eastern flank. Hundreds of thousands of Ukrainian refugees have crossed the Polish border.

It is a measure of the importance of Poland’s position on issues like the MiG-29 fighter planes that Vice President Kamala Harris will visit the country on Wednesday on a trip that will also include a stop in Romania. Ms. Harris will discuss with the leaders of both countries how NATO can support Ukraine through security, economic, and humanitarian measures “in the face of Russian aggression,” according to a White House statement.