Secretary of Defense Lloyd J. Austin III thanked Poland's willingness to continue to look for ways to assist Ukraine, but he stressed that the U.S. does not support the transfer of additional fighter aircraft to the Ukrainian Air Force at this time, said a Defense Department official. 
Pentagon Press Secretary John F. Kirby spoke at a press conference today. 
"The best way to support Ukrainian defense is by providing them the weapons and the systems that they need most to defeat Russian aggression; in particular, anti-armor and air defense," he said. 
"We, along with other nations, continue to send them these weapons and we know that they're being used with great effect. The slowed Russian advance in the north. And the contested airspace over Ukraine is evidence alone of that," he said. 
Although Russian air capabilities are significant, their effectiveness has been limited due to Ukrainian strategic operational and tactical ground-based air defense systems, surface-to-air missiles and man-portable, air-defense systems, Kirby said. 
The Ukrainian Air Force currently has several squadrons of fully mission capable aircraft, he said. 
"We assess that adding aircraft to the Ukrainian inventory is not likely to significantly change the effectiveness of Ukrainian Air Force relative to Russian capabilities," he said. 
The intelligence community has assessed that the transfer of MiG 29 jets to Ukraine may be mistaken as escalatory and could result in significant Russian reaction that might increase the prospects of a military escalation with NATO, Kirby said.  
"We are grateful for the superb support and cooperation of our Polish allies who continue to host thousands of our troops," he said, along with millions of Ukrainian refugees.  
"Polish generosity is clearly on display for the whole world to see," he added. 
"We know the Ukrainian Armed Forces, as well as average Ukrainian citizens are defending their country with great skill and bravery. We will continue to look for ways to help them do that," he said.