MOSCOW (AFP) - Russia on Thursday warned the United States against sending weapons to Ukraine, saying such action would cause "colossal damage" to ties between the former Cold War foes, even as US Secretary of State John Kerry called for Moscow to immediately commit to a ceasefire with Kiev.

Moscow is "seriously concerned" by discussions in the West about possible arms supplies to Ukraine which is battling a pro-Kremlin insurgency in the east, said foreign ministry spokesman Alexander Lukashevich, adding that any decision could "cause colossal damage to US-Russian relations".

The warning came as Kerry arrived in snowbound Kiev earlier on Thursday amid calls for Washington to begin arming Ukraine to battle Russian-backed separatists advancing in the east.

Kerry said Russia's "aggression" is the largest threat to Ukraine and called for Moscow to immediately commit to a ceasefire.

"We talked about the largest threat that Ukraine faces today and that is Russia's continued aggression in the east," Kerry said after talks with Ukraine's leaders, calling on Moscow to make "an immediate commitment, now, to a real ceasefire".

Washington has given its clearest signal yet that it is considering providing weapons to Ukraine. President Barack Obama's pick for defence secretary, Ashton Carter, told a Senate committee he would "very much incline" toward supplying arms to Ukraine.

"The nature of those arms, I can't say right now," Carter said at his Senate confirmation hearing. "But I incline in the direction of providing them with arms, including, to get to what I'm sure your question is, lethal arms."

Nato says Russia has sent weapons, funds and troops on the ground to assist the rebel advance, which scuppered a five-month-old ceasefire in eastern Ukraine where war has already killed more than 5,000 people.

Moscow denies involvement in fighting for territory the Kremlin now calls "New Russia".