WASHINGTON (AFP) - United States Vice-President Joe Biden on Tuesday blamed separatist fighters and Russian forces as he condemned the violation of a three-day-old ceasefire in eastern Ukraine and warned of increased "costs" to Russia.

Mr Biden "strongly condemned the violation of the ceasefire by separatist forces acting in concert with Russian forces, in and around the town of Debaltseve", the White House said in a statement after he spoke with Ukrainian President Petro Poroshenko.

The leaders of France, Germany, Russia and Ukraine agreed on a ceasefire deal last week in the Belarus capital Minsk to end fighting between government troops and pro-Russian rebels, which has claimed more than 5,400 lives in the past year.

But separatists on Tuesday stormed the port town of Debaltseve, a violation of the ceasefire. "If Russia continues to violate the Minsk agreements, including the most recent agreement signed on Feb 12, the costs to Russia will rise," the White House statement added.

Mr Biden and Mr Poroshenko said the ceasefire violation was confirmed by the Special Monitoring Mission of the Organisation for Security and Cooperation in Europe (OSCE).

"The Vice-President also strongly condemned Russia and the separatists for blocking access to Debaltseve by OSCE monitors, which is allowing the separatists to continue their attacks without inhibition," the White House said.

Mr Poroshenko, in a phone conversation with German Chancellor Angela Merkel, said the assault on Debaltseve, a strategic rail hub in Ukraine's east, was a "cynical attack" on the truce brokered last week.