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

President Bashar Assad rejected any immediate negotiations with the opposition, striking a further blow to already staggering international efforts for talks to end to the conflict. Assad told U.N. 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 U.N. estimates has killed more than 7,500 people. That makes it likely that the conflict will continue to edge toward civil war.

Syrian forces have 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 Assad’s regime.

Saturday morning, troops blasted Idlib for hours with dozens of tank shells as the forces moved to encircle the town, an Associated Press team in Idlib reported.

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, the AP team said. Trucks 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.