DAMASCUS, Jan. 4 (Xinhua) -- The Syrian army unleashed a counter-offensive Thursday to break a siege imposed by al-Qaida-linked militants on a key military base east of the capital Damascus, a military source told Xinhua.

The army units have started their counter-attack and captured buildings east of the municipal building in the city of Harasta in northeastern Damascus amid heavy artillery shelling and rocket fire targeting the positions of the al-Qaida-linked Levant Liberation Committee (LLC) and Failaq al-Rahman, the source said on condition of anonymity.

The aim of the operation is to drive back the militants who are besieging the Vehicle Base in Harasta, the source added.

A day earlier, tanks were seen heading from Damascus toward Harasta in anticipation of the counter-offensive of the Syrian army, just days after the ultra-radical militants unleashed their wide-scale offensive to capture that key base.

The Syrian Observatory for Human Rights also reported the counter-offensive in the vicinity of Harasta, saying the attack is coupled with intense shelling.

The London-based watchdog group said the shelling and the battles over the past seven days in that area have claimed the lives of 77 civilians.

The Vehicle Base is the largest military facility in the Eastern Ghouta region, stretching from Harasta to Arbeen.

The base contains large numbers of soldiers, including Republican Guards, as well as big weapon depots.

During the war, the base has become a major operation center and supply facility to the Syrian forces inside the Eastern Ghouta region, which contains many towns and neighborhoods.