WASHINGTON 

The US assesses a Polish proposal to transfer Soviet-era MiG-29 fighter jets to Ukraine through a US base in Germany as "high-risk," the Pentagon said Wednesday.

"We believe the best way to support Ukrainian defense is by providing them the weapons and the systems that they need most to defeat Russian aggression," said spokesman John Kirby at a press conference.

Kirby said Defense Secretary Lloyd Austin spoke to his Polish counterpart Mariusz Błaszczak earlier in the day and told him that the US does not support the transfer of additional fighter aircraft to the Ukrainian Air Force "at this time" and has “no desire to see them in our custody either."

Kirby also said the Ukrainian Air Force currently has several squadrons of fully mission capable aircraft.

"We assess that adding aircraft to the Ukrainian inventory is not likely to significantly change the effectiveness of the Ukrainian Air Force.

"Therefore, we believe that the gain from transferring those MiG-29s is low," he added.

Kirby also cited an assessment from the US intelligence community that the transfer of the jets to Ukraine "may be mistaken as escalatory and could result in a significant Russian reaction that might increase the prospects of a military escalation with NATO."

The US is currently having discussions with many countries about surface-to-air missiles, which he said the Ukrainians are more trained and more equipped to operate.

At least 516 civilians have been killed and 908 others injured in Ukraine since Russia launched a war against its neighbor on Feb. 24, according to UN figures, with the real toll feared to be higher.

More than 2.1 million people have fled Ukraine to neighboring countries, according to the UN Refugee Agency.