Kiev, Ukraine: Pro-Russian forces massing near Ukraine's port city of Mariupol are continuing to attack government troop positions, Kiev said on Monday, fuelling further concerns for the fate of an internationally brokered ceasefire.

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

Pro-Russian rebels guard a bus carrying Ukrainian prisoners of war in Luhansk. Credit:AP

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.

A withdrawal of heavy weapons from the frontline - a key step of the truce - has still not gone ahead despite both sides on Sunday agreeing to start the process.