BEIRUT – Syrian government forces attacked rebels with helicopter gunships in the heart of Damascus on Tuesday, escalating a campaign to crush their opponents as clashes spread to new areas, illustrating the rebels’ growing reach.

Cracks of gunfire and explosions echoed inside the capital for a third day, including a firefight near the country’s parliament, in an unprecedented challenge to government rule in President Bashar Assad’s seat of power.

Neighboring Iraq called on its citizens living in Syria to return home as the fighting overshadowed another round of diplomatic maneuvering to end the civil war. Special envoy Kofi Annan was in Moscow in an attempt to rescue his faltering peace plan. Plumes of gray smoke billowed over the Damascus skyline and helicopter gunships strafed the area, activists said – a sign the regime is growing desperate to push the rebels away from the heavily guarded capital.