Ukrainian forces "right now have available to them more fixed-wing fighter aircraft than they did two weeks ago," Pentagon spokesman John Kirby told reporters.

"Without getting into what other nations are providing, they (Ukrainian forces) have received additional platforms and parts to be able to increase their fleet size," he said.

Kirby did not specify the type of aircraft delivered to the Ukrainian military, which had been pleading for warplanes for weeks, but suggested that they were Russian-made.

"Other nations who have experience with those kinds of aircraft have been able to help them get more aircraft up and running," Kirby said.

He underscored that the United States, which does not want to risk being deemed a co-combatant in the conflict by Moscow, had helped with the shipment of some parts but that it has "not transported whole aircraft."

Kyiv has asked its Western partners to provide Mig-29s that its pilots already know how to fly, and which a handful of Eastern European countries have.

A possible transfer of such aircraft from Poland was discussed in early March, before the United States poured cold water on the plan, fearing Russia would see it as direct involvement of NATO in the conflict.