A ceasefire in Ukraine was cautiously observed by both sides early Sunday, despite accusations by Kiev and the US that Russia had fuelled a final push by rebels to gain territory before the deadline.
Ukrainian President Petro Poroshenko ordered troops to abide by the truce from midnight (2200 GMT), in line with a deal reached in Minsk earlier this week with the leaders of Russia, Germany and France.
No fresh fighting was reported in eastern Ukraine after the truce went into force, according to AFP reporters on the ground and official sources.
But in comments broadcast live on television, Poroshenko added that the peace process was already "threatened" by pro-Russian separatists who have surrounded Ukrainian forces in the battleground town of Debaltseve.
Meanwhile US Secretary of State John Kerry spoke by telephone with Russian Foreign Minister Sergei Lavrov to underscore "the importance of full implementation" of peace agreements, including the ceasefire.
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," a senior State Department official said.
In the hours before the ceasefire deadline, Kiev loyalist and regional police chief Vyacheslav Abroskin said constant artillery bombardments were razing Debaltseve, a strategic railway hub where Ukrainian forces were hanging on.
"There are non-stop artillery bombardments of residential areas and buildings. The town is in flames," Abroskin wrote on Facebook.
Ukraine's Azov volunteer battalion also reported fierce clashes just to the east of the vital government-held port city of Mariupol and said that the village of Shyrokyne had been "practically destroyed" by shelling.
- Deal in doubt? -
The ceasefire is the first test of the commitment by Kiev and pro-Russian separatists to the peace plan signed Thursday after marathon talks between Poroshenko, Russian leader Vladimir Putin, German Chancellor Angela Merkel and French President Francois Hollande.
Saturday's surge in fighting cast doubt on the deal but Putin was said to have re-affirmed his commitment to the ceasefire in a phone call with Merkel and Hollande.
"Putin said the rebels were ready for the ceasefire," the French presidency said in a statement.
Donetsk rebel chief Alexander Zakharchenko -- viewed by the West as a Russian puppet -- earlier ordered his fighters to respect the truce but to ward off any attacks "with all force and means".  
The fragility of the truce was further highlighted when Zakharchenko warned that any attempts by encircled Ukraine troops to escape Debaltseve after the ceasefire will be viewed as an act of aggression.
The UN Security Council is expected to meet on Sunday for an emergency session to shore up the Ukraine peace deal, diplomats said.
- Last push before truce -
US President Barack Obama called his Ukrainian counterpart Saturday to express his "deep concern" over the spiralling violence ahead of the midnight deadline.
"Both leaders stressed the importance of establishing a lasting peace that respects Ukraine's sovereignty and unity," the White House said in a statement.
Obama has warned he could start arms supplies to Ukraine if the new peace deal collapses.
The fragile accord is seen as the best hope of ending the conflict, which has killed at least 5,480 people and ratcheted up East-West tensions to levels not seen since the Cold War, but scepticism remains high after the previous collapse of a similar peace plan.
In the run-up to the ceasefire, the United States said the Russian military had deployed large amounts of artillery and multiple rocket launcher systems and was using them to shell Ukrainian positions.
Ukrainian security spokesman Andriy Lysenko said rebels backed up by regular Russian troops were trying to "achieve important tactical goals to extend their territory" before the ceasefire.
Lysenko said seven soldiers were killed and 23 wounded in clashes over the past 24 hours, while rebel and government officials said six civilians had died.
- Roadmap to peace -
The new Minsk agreement is fraught with potential pitfalls.
Both sides have to begin withdrawing heavy weapons from the front line within two days of the start of the ceasefire to establish a buffer zone between 50 and 140 kilometres (31-87 miles) wide, depending on the range of the weapons.
Under the Minsk agreement, Kiev will also begin retaking control over the approximately 400-kilometre (250 mile) stretch of Russia's border with rebel-held Ukraine, but only after local elections are held.
The border is entirely under Russian and rebel control and is used, according to Kiev, as a conduit for separatist supplies.
Separatist-held territories will be granted a degree of autonomy to be established through talks.
Source: AFP