Evacuation plans are in place in a government-backed town in eastern Ukraine that has seen an upsurge in violence which left at least eight people dead.
The spike in artillery use in Avdiivka, right by the separatist-controlled Donetsk, forced the president, Petro Poroshenko, to cut short a meeting with the German chancellor, Angela Merkel, on 30 January.
A day later, shelling damaged an electricity station at Zasyadko coal mine which cut out its elevators and left 200 workers trapped underground for several hours. The town's estimated 22,000 people have been left without electricity for days.
Spokesman for the separatists, Eduard Basurin, denied separatist shelling, said to be the heaviest for months, had cut power lines which he insisted had been damaged earlier.
Chairman of the Ukrainian Security and Defence Council, Oleksandr Turchynov, said the shelling around killed at least three troops and injured 24 more, the Associated Press reported.
Kiev says the rebels refused to cease fire to remove the bodies of the dead and wounded. The rebels' Donetsk News Agency says four rebel fighters had died and seven were injured.
Head of the local administration Pavlo Zhebrivsky, said: "As of now, we can evacuate up to 8,000 people in the course of the day. The region's towns are ready to receive up to 9,000 people," Interfax news agency reported.
Kremlin spokesman Dmitry Peskov said it had "reliable information" that Ukrainian volunteer battalions crossed the front line and tried to capture rebel territory.
Concerns are growing in Kiev about what a Trump administration will mean for ties between the US and Ukraine, particularly as the spike in violence comes just after a phone call between Donald Trump and Vladimir Putin.
Previously very supportive of Ukraine, Kiev fears it may lose that support. The former Nato chief General Sir Richard Shirreff said in January that resolving the conflict was not a matter for the alliance but required help from individual nations.