US military leaders said Wednesday that the White House was still mulling whether to arm Kiev with lethal weapons, and admitted that Russian response to such lethal aids was unpredictable.

Air Force General Philip Breedlove, military chief of the North Atlantic Treaty Organization (NATO), told the House Armed Services Committee that in his discussion with the US President Barack Obama about possible military options, he introduced a wide range of weapons required by the Ukrainian forces, from small arms to more sophisticated ones which require advanced training.

The focus of the discussion was "to give Ukraine more ability to defend itself against the separatist aggression" instead of fundamentally altering the military balance, said Gen. Breedlove, adding that equipped with lethal aids, Kiev may change "Russian calculations".

However, Breedlove acknowledged that the US military had found no way to be sure how Moscow would react if US supplies of lethal aids reach Kiev.

"Clearly we don't know what (Russian President Vladimir) Putin will do," said Breedlove. "It could cause positive results. It could cause negative results."

Breedlove's concerns were echoed by another witness Christine Wormuth, undersecretary of defense for policy, who said Russia " might then double down" and escalate the conflicts.