Pro-Russian forces massing near Ukraine’s port city of Mariupol are continuing to attack government troop positions, Kiev says, fuelling concerns for the fate of an internationally brokered ceasefire.

Continued hostilities there and elsewhere meant a pull-back of heavy weapons could not go ahead as agreed, Ukrainian officials said.

“As Ukrainian positions are still being fired upon there can be no talk yet of a withdrawal of arms,” military spokesman Vladyslav Seleznyov wrote in a statement on Facebook on Monday.

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Tensions were also high following a bomb blast Sunday in the normally peaceful eastern city of Kharkiv. In their latest toll, authorities said that three people had died in the “terrorist” attack.

The West has warned of additional sanctions on Russia if the shaky truce should deteriorate further, especially after rebels last week took the strategic town of Debaltseve in defiance of the ceasefire meant to start February 15.

The Organisation for Security and Cooperation in Europe, tasked with monitoring the truce, “concludes that the ceasefire is not holding in critical, strategic points,” including near Mariupol and in Debaltseve, the deputy head of the OSCE mission, Alexander Hug, told France 24 television.

A Ukrainian military commander, Colonel Valentyn Fedichev, said Monday that, while the number of attacks had generally decreased across the conflict zone, troop positions were still fired on 27 times since Sunday. Two Ukrainian soldiers were killed and 10 wounded, he said.

Insurgent fighters “have not halted attempts to assault our positions in the town of Shyrokine and the Mariupol area,” Fedichev said.

Other defence officials said the rebels fired mortars into Shyrokine, which neighbours Mariupol, in an apparent attempt to provoke troops into firing back in violation of the ceasefire.

Kiev has alleged Russia sent 20 tanks towards Mariupol, a port city of half a million residents on the Azov Sea coast, and that two tank attacks occurred there on Sunday.

Moscow denies giving military support to the rebels. However it made the same denials over Crimea – the Ukrainian Black Sea peninsula that it annexed last year – before finally admitting that it had deployed troops.

If Mariupol were to fall to the pro-Russian rebels, it would remove a key obstacle to creating a separatist land corridor stretching from Russia’s border with Ukraine to Crimea.

The United States and the European Union, however, have strongly warned against further breaches to the ceasefire, with Washington saying extra sanctions could be imposed on Russia within days.