Beirut Syrian troops launched a fierce assault on a Damascus suburb Thursday, days ahead of a deadline for a UN-brokered ceasefire, with activists describing it as one of the most violent attacks around the capital since the year-old uprising began.

The operation in Douma, along with other offensives around the country, bolstered the opposition's claim that President Bashar Al Assad is only intensifying violence ahead of the Tuesday deadline to implement the truce. Activists say Al Assad wants to make gains on the ground before the ceasefire is supposed to take effect.

There are other signs as well that the government has no intention of abiding by the truce deal brokered by former UN chief Kofi Annan.

The pro-government daily Al Watan quoted an unnamed official saying the government is not bound by Tuesday's deadline for a ceasefire because that day marks "the beginning of army units' withdrawal and not the end. It is not a deadline by itself".

In Geneva, Annan's spokesman Ahmad Fawzi said both sides are expected to end the hostilities within 48 hours of the April 10th deadline.

"The clock starts ticking on the 10th for both sides to cease all forms of violence," he said. Fawzi added that Syria has told UN officials it is withdrawing troops from the southern province of Daraa, the northwestern province of Idlib and the mountain resort town of Zabadani, north of Damascus.

Checkpoints

But Mohammad Fares, an activist in Zabadani, denied claims that troops withdrew and said the army is still present in the town with checkpoints backed by tanks.

"Troops and tanks are in Zabadani and around it," he said by telephone.

Other activists reported attacks on both Daraa and Idlib on Wednesday.

Douma-based activist Mohammad Saeed reported that troops shelled residential areas yesterday with tanks "in one of the most violent campaigns against the area since the uprising started." He said troops were using detainees as human shields as they marched into one of the suburb's main squares, a few kilometres northwest of Damascus.

"Soldiers in the Ganam Square near the vegetable market were walking behind detainees," Saeed said via Skype. "They do that so that members of the Free Syrian Army do not open fire at the troops."

He said the shelling and clashes have been going on since 7am. "The regime has no limits in its crimes. They are ready to do anything," said Saeed. "They are hitting homes directly."

The Britain-based Syrian Observatory for Human Rights also reported one of the biggest operations of the uprising in Douma, where army defectors are believed to be active.

Amateur videos posted by activists showed black smoke billowing from residential areas as heavy cracks of gunfire sounded.

Douma, which has witnessed anti-Al Assad activities since the uprising began, has been subjected to several campaigns by Al Assad's regime over the past year.