BEIRUT - Fierce clashes between Syrian government forces and rebel fighters erupted on Thursday in Douma, near the capital Damascus, while violence also raged in other parts of the strife-torn country, monitors said.

The Syrian Observatory for Human Rights said regime troops backed by tanks stormed Douma at dawn amid heavy gunfire and shelling.

The Britain-based group said clashes between soldiers and rebel forces were reported in various parts of the city, located some 13 kilometres (seven miles) northeast of the capital.

Plumes of smoke could be seen rising near the city's main mosque as troop reinforcements were sent in, the Observatory said.

Violence across the country has left at least 170 people dead so far this week despite a pledge by President Bashar al-Assad to implement a peace plan brokered by UN-Arab League envoy Kofi Annan.

Two young men were killed in Kfar Sousa, a neighbourhood of Damascus early Thursday when security forces opened fire on their car, the Observatory said.

Activists added that troops were also in pursuit of several other young men in the neighbourhood.

Five soldiers were meanwhile killed in two separate attacks in the northern province of Aleppo and in southern Daraa, cradle of the revolt that broke out last year in March against Assad's regime.

Clashes were reported in several towns of Aleppo province, including Andan and Hritan where messages on loudspeakers urged regime troops to defect and join the opposition, the Observatory said.

Washington on Wednesday criticised the "intensification" of violence against opponents of the regime given Assad's pledge to implement by April 10 Annan's six-point peace plan.

"We've yet to be convinced that they have any intention of complying with the April 10 deadline," US State Department spokesman Mark Toner told reporters.

The unrest in Syria has left more than 9,000 people dead since mid-March last year, according to UN figures.

More than 1,000 Syrian refugees fled to Turkey in the last 24 hours, bringing to nearly 21,000 the total number of people who have crossed to escape a deadly crackdown, a Turkish official said on Thursday.

"A total of 1,043 Syrians made their way to Turkey in a single day, marking the highest figure in recent times," the official said on condition of anonymity.

The revolt against the regime began as a popular uprising but has transformed into an insurgency many fear will lead to a full-blown civil war.