Syria has launched a long-anticipated assault to crush the opposition in the rebellious north, bombarding its main city with tank shells from all sides and clashing with rebel fighters struggling to hold back an invasion.

President Bashar Assad meanwhile rejected any immediate negotiations with the opposition, striking a further blow to already staggering international efforts for talks to end to the conflict.

Mr Assad told UN envoy Kofi Annan that a political solution is impossible as long as "terrorist groups" threaten the country.

The opposition's political leadership has also rejected dialogue, saying talk is impossible after a yearlong crackdown that the UN estimates has killed more than 7,500 people.

That makes it likely that the conflict will continue to edge toward civil war.

Syrian forces had been building up for days around Idlib, the capital of a hilly, agricultural province along the Syria-Turkey border that has been a hotbed of protests against Mr Assad's regime.

Yesterday morning, troops blasted Idlib for hours with dozens of tank shells as the forces moved to encircle the town.

Families fled their homes, carrying blankets and a few other meager belongings. Others huddled in homes.

Rebel fighters rushed through Idlib's streets, taking cover behind walls to fire on the attackers with automatic weapons.

Lorries sped wounded fighters to clinics, and men on one street destroyed speed bumps with shovels so ambulances could drive faster.

Many low-level soldiers in the area have joined the opposition and fight along with civilians who have taken up arms as part of the loosely organised Free Syrian Army.

Many fear the offensive in Idlib could end up like the regime's campaign against a rebel-held neighbourhood in the central city of Homs.

Troops besieged and shelled Baba Amr for weeks before capturing it on March 1.

Activists said hundreds were killed, and a UN official who visited the area this week said she was "horrified" by the destruction in the district, now virtually deserted.