KIEV (AFP) - A two-day-old truce in Ukraine was under threat Monday as fighting raged around a strategic railway hub and other violations were reported, jeopardising an agreement to start withdrawing heavy weapons from the frontline.

The European Union, which backed the new peace effort brought about last week through painstaking mediation by France and Germany, pleaded that "the shooting needs to stop".

The bloc also imposed fresh sanctions on Russia, which it accuses of militarily backing the pro-Russian rebels fighting Ukraine's government troops in a bid to carve out separate republics in the east.

Russia, which denies involvement in the conflict, vowed an "appropriate response" to the EU move.

Kiev officials and rebels accused each other of ongoing attacks that prevented them from pulling back tanks, rockets and artillery from the frontline in Ukraine's east, near the insurgent-held cities of Donetsk and Lugansk.

The withdrawal of heavy weapons is meant to begin by midnight Monday (6 am Singapore time Tuesday).

The worst of the violence was around the town of Debaltseve, a key transport hub between Donetsk and Lugansk, where thousands of government troops were mostly surrounded by heavily armed rebels.

"Non-stop explosions" were heard there on Monday, said Natalia Karabuta, a municipal official who fled the town. She added that around 5,000 civilians were trapped inside Debaltseve with little food and water.

An Organization for Security and Cooperation in Europe (OSCE) team tasked with monitoring the ceasefire has been unable to enter Debaltseve because of the ongoing hostilities.

A Ukrainian military spokesman, Vladyslav Seleznyov, said "there is no question at the moment of us withdrawing heavy weapons" from the frontline because of rebel attacks.

The rebels also rejected pulling back any heavy weapons on their side until a "full" ceasefire was in effect.

Both sides accused the other of renewing attacks in some other areas, including at rebel-held Donetsk airport and near the port city of Mariupol.

Ukraine's government said the rebels had launched more than 112 assaults on its troops' positions since the ceasefire started Sunday, killing five soldiers and wounding 25.

European Commission spokeswoman Maja Kocijancic said in Brussels that while the truce was seen as "largely" holding, "it is imperative the ceasefire is fully implemented... it means the shooting needs to stop".

German Chancellor Angela Merkel echoed that call while admitting "the situation is fragile".

"I always said that there was no guarantee that we would succeed in our efforts. It is an extremely difficult path," she said in Berlin.

Merkel and French President Francois Hollande together brokered the deal that last Thursday paved the way for the ceasefire.

The ultimate aim of the peace plan is to end the 10-month-old conflict that has killed more than 5,600 people, according to the UN.

A previous ceasefire agreed in September stalled within days and eventually disintegrated.

Earlier Monday, the EU announced new sanctions on Moscow, adding two Russian deputy defence ministers and two Russian lawmakers and another Russian citizen to its travel-ban and asset-freeze blacklist. Fourteen Ukrainian rebel military or political figures were also blacklisted, along with nine organisations.

Europe and the US have dismissed Russian President Vladimir Putin's denials of having a hand in Ukraine's eastern insurrection.

They have noted that he made similar protestations over Ukraine's Crimea peninsula before finally admitting he had ordered in Russian troops there ahead of annexing the territory last year.

Russia's foreign ministry on Monday lashed out at the latest EU sanctions as "inconsistent and illogical".

"Such decisions... will be followed by an appropriate response," it warned in a statement.

Russia has already banned European food imports in retaliation for previous sanctions.