This is reported by RIA Novosti. 

“Ukrainians use MiG and Sukhoi aircraft, while the United States does not have either,” the Pentagon said.

Washington added that "we were able to help the Ukrainians with certain spare parts and other equipment in order to support the existing fleet."

“But we did not transfer any American aircraft,” a US Department of Defense spokesman said.

He also said that the United States is not currently training Ukrainian pilots.

Earlier, John Kirby, strategic communications coordinator at the White House National Security Council (NSC), said that Pentagon specialists are preliminary studying the possibility of supplying fighter jets to the Ukrainian army, but such deals are impossible in the short term.