By Anton Zverev
NIZHNYAYA KRYNKA, Ukraine (Reuters) - A senior pro-Russian rebel commander said separatist forces were due to begin withdrawing heavy weapons from the front line in east Ukraine on Sunday, a sign rebels may be prepared to halt their advance as part of an internationally brokered peace deal.
Fighting has eased in many areas since a ceasefire came into effect a week ago, but the truce was shaken by the rebel capture on Wednesday of the town of Debaltseve, forcing a retreat by thousands of Ukrainian troops.
But moves towards pulling back heavy weapons, together with the exchange of dozens of prisoners with Ukrainian government forces on Saturday, could indicate the rebels intend to observe the truce more fully, having achieved a key military objective by seizing Debaltseve.
"The plan was signed last night ... Starting from today there are two weeks to withdraw heavy weapons," Interfax news agency quoted rebel commander Eduard Basurin as saying.
Russian news agency TASS quoted him as saying the pull-back was still being organized and that the actual withdrawal of weapons would take place from Tuesday.
Ukrainian military spokesman Andriy Lysenko said by telephone there was no confirmation yet as to whether the rebels had started pulling back their weapons.
A Reuters witness saw a 20-vehicle convoy of separatist military trucks with anti-aircraft missile systems and howitzers leaving Debaltseve in the direction of Donetsk.
On Saturday, government forces and rebels exchanged nearly 200 prisoners, one of the first moves to implement the peace deal reached on Feb. 12 in the Belarussian capital Minsk after the French, German, Russian and Ukrainian leaders met.
Both sides continue to accuse each other of violating the ceasefire.
The Ukrainian military said rebels had launched 12 attacks on government troop positions overnight, using artillery and mortar fire.
The town of Pesky near Donetsk had seen the most intense fighting, while separatist groups had attempted to "storm" Ukrainian positions in Shyrokyne, east of the port city of Mariupol on the Sea of Azov, the military said on Facebook.
Kiev accuses the separatists of building up forces and weapons in Ukraine's southeast and has said it is braced for the possibility of a rebel attack on Mariupol.
The rebel press service DAN said Ukrainian troops had been shelling parts of Donetsk, reporting that artillery fire could be heard in the city at around 0730 GMT on Sunday.
(Additional reporting by Natalia Zinets in Kiev, Alexander Winning in Moscow; Writing by Alessandra Prentice; Editing by Jason Neely)