The US has accused Russia of violating the Minsk agreement on Ukraine, as the UN Security Council voted unanimously to approve the ceasefire deal.
Vice-President Joe Biden said “the costs to Russia will rise” if it continued to violate the accord.
Fighting is continuing around the strategic town of Debaltseve, with pro-Russian rebels saying they now control most areas.
Russian President Vladimir Putin urged Ukraine’s troops there to surrender.
Although the Security Council unanimously approved a Russian-drafted resolution to endorse the Minsk ceasefire deal agreed in Belarus last week, angry words were exchanged among ambassadors.
US Ambassador Samantha Power welcomed the agreement but said that Russia had to prove its commitment to peace.
She said: “Stop arming the separatists. Stop sending hundreds of heavy weapons across the border in addition to your troops. Stop pretending you are not doing what you are doing.”
She added: “Russia signs agreements then does everything within its power to undermine them. Russia champions the sovereignty of nations and then acts as if a neighbour’s borders do not exist.”
Russian Ambassador Vitaly Churkin called her comments “offensive
“Since the very start of the crisis, Russia has actively called for a peaceful settlement through inclusive, transparent dialogue between all sides in the internal Ukrainian conflict,” he said.
After speaking to Ukrainian President Petro Poroshenko, Mr Biden said he “strongly condemned the violation of the ceasefire by separatist forces acting in concert with Russian forces, in and around the town of Debaltseve”.
He added: “If Russia continues to violate the Minsk agreements… the costs to Russia will rise.”
Mr Poroshenko described rebel attempts to take the town as a “cynical attack” on the ceasefire.
International observers monitoring the truce have been unable to enter Debaltseve.
The town has become a key prize for rebels and government forces, as it sits on a strategic railway line linking rebel-held Donetsk and Luhansk.