The ceasefire deal brokered by European leaders between Ukrainian forces and pro-Russian separatists is on the brink of collapse as rival sides have stopped pulling back their heavy weapons from the restive frontline of the conflict.

Despite the ceasefire taking effect over the weekend, fighting in sensitive areas in the eastern Ukrainian region continues.

The town of Debaltseve, besieged by the anti-Kiev rebels, remains the flashpoint of the latest violence, which threatens to leave the painstakingly hammered out ceasefire in tatters.

Ukraine says the rebels, accused of being backed by Moscow, fired on its troops more than 100 times in the last 24 hours. Kiev says the attack killed five Ukrainian troops and left 25 others injured.

"The withdrawal [of heavy weapons] will begin once there is a ceasefire," said Andriy Lysenko, spokesperson for Ukraine's National Security and Defence Council.

The rivals were given two days to withdraw the heavy weapons after the ceasefire came into force on Sunday, 15 February midnight.

Separatist leaders have, however, pointed fingers at the Ukrainian side and called on Kiev to withdraw the heavy weapons.

Expressing concern over the escalation in Debaltseve, US State Department spokesperson Jen Psaki said: "These aggressive actions and statements by the Russia-backed separatists threaten the most recent ceasefire. We call on Russia and the separatists it backs to halt all attacks immediately."

However, the ceasefire is holding in many other parts of eastern Ukraine.

"Today's information is unique...Not a single shot [in Luhansk]. Well, at least I don't have information that there was," Ruslan Tkachuk, a spokesperson representing the self-proclaimed Luhansk People's Republic told 112 Ukraina television channel.

Russian President Vladimir Putin, German Chancellor Angela Merkel and Ukrainian President Petro Poroshenko have also held a telephone conversation over the situation.