DAMASCUS, Jan. 28 (Xinhua) -- The rebel groups, including al-Qaida-linked ones, launched a fresh offensive Sunday against a key Syrian military base in the eastern countryside of the capital Damascus, as part of the rebels' determination to capture that strategic base in Harsta city, activists said.

The attack started when the rebels sent an explosive-laden vehicle, targeting the Vehicle Management base in the city of Harasta in Eastern Ghouta.

The sound of the explosion was clearly heard in the eastern part of Damascus.

The activists said the vehicle tore through the base, while the War Media, the media wing of the Syrian army, said the Syrian forces in that base targeted the vehicle before reaching its destination amid an ongoing military showdown in Harasta.

Activists said the rebels of the Levant Liberation Committee (LLC), previously known as the al-Qaida-linked Nusra Front, as well as other rebel groups, launched the third phase of the attack against the Vehicle Management base in Harasta.

The first wave of the attack was on Jan. 1 when the rebels unleashed a surprise attack and managed to lay a siege on that key base as a prelude to storming it.

A few days later, the Syrian army sent reinforcements and launched a counter-offensive, succeeding to break the siege on that base from one direction.

But Sunday's escalation comes as at a time when an alleged ceasefire went into effect at midnight Saturday after an agreement was reportedly reached between the government and the opposition during the intra-Syrian talks in Vienna on Thursday and Friday.

The ceasefire agreement was reported by several Arab and foreign media outlets, but the state Syrian media remained tight-lipped.

The escalation in Harasta also comes a day ahead of the Russian-backed Syrian talks in Sochi, which is designed to find a common ground for a solution in Syria's nearly seven-year war.

Now, the sound of warplanes over Damascus is clearly heard, as activists are reporting intense shelling and airstrikes by the Syrian army on Harasta amid fears that the rebels will respond by firing mortar shells on the residential areas in the capital as they usually do.

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

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.

The LLC was designated as a terror group by the UN. The group later attempted to polish its image by announcing cutting ties with al-Qaida as well as changing its name, but it is still viewed as a terror group.