BEIRUT, Syria (AP) — Riding a wave of momentum, Syrian rebels made a run on Aleppo yesterday in some of the fiercest fighting seen in the country's largest city, which has been a key bastion of support for President Bashar Assad over the course of the 17-month-old uprising.

The rebels also took over a third border crossing — and the second one along Syria's northeastern frontier with Iraq — another sign the regime's tight grip on the country is wobbling.

The fighting in Aleppo comes on the heels of intense clashes in the capital, Damascus, as rebel forces target the pillars of regime power in their attempts to usher in what they hope will be the end of Assad's rule.

"There were huge explosions and the gunfire didn't stop for several hours," Aleppo-based activist Mohammad Saeed told The Associated Press via Skype. "The uprising has finally reached Aleppo."

The city has remained largely loyal to Assad and been spared the kind of daily bloodshed that has plagued other areas.

But Saeed said dozens of fighters from the rag-tag Free Syrian Army entered Aleppo — a commercial hub — from the countryside and were fighting regime troops from inside.

It was the first sustained fighting in the city centre, focused on the Salaheddine district, although there have been protests in Aleppo and violence on the outskirts.

The rebels have put the regime on the defensive after a week of battles in the capital, Damascus, including a bombing that struck at the heart of the regime, killing four high-level government officials. The coming days will be crucial to determining whether the regime can recover from the blows, which have punctured the sense that Assad's hold on the country is impenetrable.

Rebels also took over the Syrian side of the border crossing at the Iraqi town of Rabiya, 520 kilometres (320 miles) northwest of Baghdad, according to Atheel al-Nujaifi, the governor of Iraq's Ninevah province.

Iraqi officials said rebels tore up and shot at posters of Assad but did not face any resistance from regime authorities who surrendered the sprawling, dusty border crossing peacefully. Iraqi officials quickly barricaded the crossing and ordered additional troops to secure the area.