UN backs ceasefire deal, US warns Russia as fighting continues in east Ukraine
The ceasefire, which came into effect on Sunday, has been broadly observed but separatists insist the agreement does not apply in Debaltseve because they have the town almost surrounded.
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.
US Ambassador Samantha Power said she "wholeheartedly welcomes this 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."
Ms Power said it was "ironic" Russia had drafted the resolution while "backing an all-out assault" in Ukraine.
Meanwhile, sources in the self-proclaimed Donetsk People's Republic (DPR) said Debaltseve police station and railway station had been taken, and at least 80% of the city was under rebel control.
According to later reports, the city's military HQ - where many government troops are based - has also been surrounded.
Most of its 25,000 population has been evacuated but about 7,000 civilians are still believed trapped by the fighting.
The ceasefire, which came into effect on Sunday, has been broadly observed but separatists insist the agreement does not apply in Debaltseve because they have the town almost surrounded.
Both sides have also failed to pull back heavy weapons from the front line.
The withdrawal was due to start no later than the second day after the truce came into effect and be completed within two weeks, creating buffer zones 50-140km (30-85 miles) wide.
The UN says more than 5,600 people have been killed since the conflict erupted in eastern Ukraine in April, but there are fears the actual death toll could be much higher.
