Pro-Russian rebels have taken Ukrainian troops prisoner as they seized control in parts of the key transport hub of Debaltseve.

Ukraine's defence ministry admitted some soldiers had been captured after an ambush - but said the number was lower than the 300 stated by Russian media.

While the internationally brokered ceasefire that came into force on Sunday is holding elsewhere in eastern Ukraine, fighting continues in Debaltseve, where thousands of government troops had been encircled by rebels.

The defence ministry said: "Street fighting is continuing, rebels are attacking the town with assault groups backed by artillery and armoured vehicles.

"Part of the town has been seized by the bandits."

It is not clear how much of Debeltseve is under the control of separatist forces.

"Eighty percent of Debaltseve is already ours," claimed Eduard Basurin, a rebel leader. "A clean-up of the town is under way."

Five Ukrainian soldiers have been killed and 14 wounded in clashes with rebels.

The fighting is the latest apparent breach of a fragile ceasefire under which both sides were supposed to pull heavy artillery back from the front line today.

The ceasefire, agreed after talks between Russian, Ukraine, Germany and France, required both sides to withdraw heavy artillery from the front line on Tuesday.

This has not yet happened.

The rebels have declared they will begin withdrawing heavy weaponry from those areas of the front line where the ceasefire holds, but this does not appear to include Debaltseve.

Rebel leader Andrei Purgin said the separatists were prepared to discuss withdrawing with the international group monitoring the ceasefire, the Organisation for Security and Cooperation in Europe (OSCE).

The leaders of Ukraine, Russia and Germany discussed the ceasefire in a phone call late last night.

During the call German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko appealed to Russian President Vladimir Putin to use his influence on the separatists to ensure that they stop fighting.

Meanwhile, the US said it was "gravely concerned" about the fighting in Debaltseve.

More than 5,000 people have been killed since the conflict began last April.

Originally published at 11.56am