The Pentagon said that the assignment of 300 "Sky Soldiers" – the proud name for members of the Italy-based 173rd Airborne Brigade Combat team – to conduct the training was a significant upgrade from earlier U.S. military exercises with Ukrainians, which previously were undertaken by National Guard units.

"This is certainly an uptick in the training," said Army Col. Steve Warren, a Pentagon spokesman.

The exercise had been denounced by Russia, which said it risked escalating the Ukrainian conflict. The exercises place U.S. and Russian troops 770 miles apart in a country the size of Texas.

"It's a big country," Warren said. "We don't believe we are endangering U.S. soldiers."

The 173rd Airborne Brigade, an elite airborne infantry unit based on an Italian military base in Vicenza, is the U.S. Army's main quick-response force in Europe.

The California National Guard has provided more limited military training for Ukraine since 1999, eight years after the former Soviet republic declared independence as the Soviet Union dissolved.