More than 10 people have been killed in some of the worst fighting for weeks in eastern Ukraine.
Seven Ukrainian soldiers died as clashes broke out between the army and pro-Russian rebels in and around the frontline town of Avdiivka.
Fighting erupted despite an attempt to renew a ceasefire last month.
A rebel commander known as Grek and two other rebel fighters were killed. Unconfirmed reports also spoke of two civilian fatalities.
A woman was killed by shelling in the rebel-held town of Makiyivka, local sources said.
More than 9,700 people have died since the conflict erupted in 2014, as Russia annexed the Ukrainian peninsula of Crimea and pro-Russian rebels launched an insurgency in the east.
A ceasefire was eventually agreed in February 2015 but there have been frequent violations. The latest truce began on 23 December.
Ukrainian forces say the fresh outbreak of fighting began when rebels launched an attack on Avdiivka, a flashpoint town held by the army that borders land controlled by the separatists.
“The enemy continues to fire at our positions with heavy artillery and mortars,” said military spokesman Oleksandr Motuzyanyk. Electricity and water supplies are said to have been cut off.
The US and EU imposed sanctions on Russia in response to its actions in eastern Ukraine. Russia has denied backing the rebels.
The renewed violence coincided with President Donald Trump’s first phone-call with Russian President Vladimir Putin since he took office.
According to the Kremlin, the two presidents agreed to a “partner-like co-operation” on issues including Ukraine, which has caused concern in Kiev.
President Petro Poroshenko was due to discuss the conflict in Berlin on Monday with German Chancellor Angela Merkel.
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