Syrian troops are attacking a suburb of the capital Damascus in what activists described as one of the most violent campaigns since the year-old uprising began.
The Britain-based Syrian Observatory for Human Rights says "a major operation" is under way in the suburb of Douma, with tanks shelling the area and clashes between troops and army defectors.
Douma-based activist Mohammed Saeed said the operation is "one of the most violent campaigns against the area" since the uprising against President Bashar Assad's regime began in March last year.
The Observatory also said troops are clashing with rebel army defectors in the northern towns of Hraytan and Anadan near Aleppo, and there is fighting in the southern village of Kfar Shams.
The latest violence comes despite Syrian president Bashar Assad's agreement to implement a ceasefire starting on April 10.
The truce is the keystone of a plan put forward by Kofi Annan, the joint UN-Arab League envoy, aimed at ending the country's year-long crisis.
Meanwhile the spokesman for UN-Arab League envoy Kofi Annan says Syria is claiming that it has already begun troop withdrawals in some areas.
Ahmad Fawzi says officials with Assad's government "have told us they have begun withdrawing troops from certain areas" of Syria, including Daraa, Idlib and Zabadani.
Fawzi told reporters in Geneva that a Syrian ceasefire is expected to begin no later than 48 hours after April 10.
He says "the clock starts on the 10th for both sides to cease all forms of violence."
Saeed said troops were using detainees as human shields as they marched into one of the suburb's main squares, a few miles north-west of Damascus.
"Soldiers in the Ghanam 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."
In Douma amateur videos posted by activists showed black smoke billowing from residential areas as heavy cracks of gunfire sounded.
The opposition is deeply sceptical that Assad will live up to his commitment to a truce and accuses him of trying to manipulate it to buy more time to continue his military crackdown on the revolt.
The Annan plan requires regime forces to withdraw from towns and cities, followed by a withdrawal by rebel fighters. Then all sides are supposed to hold talks on a political solution.
As the fighting raged in the north, more Syrians fled to neighbouring Turkey where the country's disaster management agency said more than 1,600 refugees arrived yesterday and today.