WASHINGTON (AFP) - US President Barack Obama called his Ukrainian counterpart Petro Poroshenko on Saturday to express his "deep concern" and "sympathy" over spiraling violence as a ceasefire came into force in eastern Ukraine.
The truce officially started at midnight (6am on Sunday, Singapore time) but surging fighting in the run-up to the ceasefire has already cast doubt on whether it will be respected.
"The President spoke with Ukrainian President Poroshenko today to express his sympathy for the mounting toll of the conflict in eastern Ukraine and his deep concern about the ongoing violence, particularly in and around Debaltseve," a White House statement said.
A senior State Department official said separately that top US diplomat John Kerry spoke with Russian Foreign Minister Sergei Lavrov in which he "underscored the importance of full implementation" of peace agreements, including the ceasefire.
Mr Kerry also "expressed concern about the fierce fighting around Debaltseve, and efforts by Russia and the separatists to cut off the town in advance of the ceasefire," the official added.
Earlier, Kiev-loyal regional police chief Vyacheslav Abroskin said constant artillery bombardments were razing the strategic railway hub of Debaltseve, where Ukrainian forces were hanging on.
The State Department released images it said showed that Russia was still deploying heavy weapons in eastern Ukraine.
"The United States government considers these commercial images as one of several pieces of credible evidence that lead to the conclusion that the Russian military has deployed large amounts of artillery and multiple rocket launchers around Debaltseve, where it is shelling Ukrainian positions," spokesman Jen Psaki said in a statement.
"We are confident that these are Russian military, not separatist systems."
Mr Obama and Mr Poroshenko "emphasised the pressing need for all signatories to implement the ceasefire and protocol agreements reached at Minsk last September and reaffirmed by the Minsk Implementation Plan this week", the White House statement added.
"Both leaders stressed the importance of establishing a lasting peace that respects Ukraine's sovereignty and unity."
The US President separately spoke by phone with German Chancellor Angela Merkel, who has played a critical role in peace negotiations.
The two leaders are "particularly concerned about the intense fighting in and around Debaltseve", the White House said, adding that Mr Obama praised the Chancellor's "tireless efforts to bring the conflict in eastern Ukraine to an end in manner that preserves Ukraine's sovereignty and unity."
Under the terms of the ceasefire deal, inked on Thursday after marathon talks in Minsk between the leaders of Ukraine, Russia, Germany and France, the two warring sides have two days from the start of the truce to start pulling back heavy weapons from the front line.
The last-ditch peace plan is seen as the best hope of ending the violence that has claimed at least 5,480 lives since April but skepticism remains high after the collapse of a similar previous deal.
Ferocious fighting raged in the hours before the ceasefire was set to come into effect, with Ukraine and the United States accusing Russia of piling in weapons to fuel a rebel onslaught to grab territory.