KIEV/DONETSK, Ukraine Feb 12 (Reuters) - Ukrainian government forces and pro-Russian separatists have exchanged dozens of prisoners in east Ukraine, a Ukrainian Security Council aide confirmed on Sunday, a step towards implementing an internationally brokered peace deal.

Reuters reporters in the village of Zholobok, 20 km (12 miles) west of the rebel stronghold of Luhansk, saw more than 130 Ukrainian servicemen being released late on Saturday evening in exchange for 52 rebel fighters.

The exchange is 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.

The Security Council's Markian Lubkivskyi published a list of the 139 released Ukrainian servicemen and said the government would do its utmost to free those remaining in rebel captivity, in a post on his Facebook page early on Sunday.

Fighting has eased in many areas since a ceasefire came into effect a week ago, but the truce was badly shaken by the rebel capture on Wednesday of the strategic town of Debaltseve, forcing a retreat by thousands of Ukrainian troops.

Kiev accuses separatists of building up forces and weapons in Ukraine's south east and the Ukrainian military said on Saturday it was braced for the possibility of a rebel attack on the port city of Mariupol.

Nevertheless, rebel leaders said on Saturday they had signed a document detailing a plan for the withdrawal of heavy weapons, as required by the Minsk agreement, a sign they may be prepared to halt their advance, having achieved their main military objective by seizing Debaltseve.