Syrian warplanes on Sunday bombed the Baba Amr district of Homs hours after insurgents attacked the former rebel stronghold that was devastated by an army siege last year, a watchdog said.

"The air force is bombing Baba Amr," which thousands of residents have returned to since it was taken by the army last March, said Syrian Observatory for Human Rights director Rami Abdel Rahman.

Rebels had launched a surprise dawn attack on the battered Homs neighbourhood for the first time since they were driven out by the army a year ago in a bloody campaign that lasted more than a month and left hundreds dead.

The Observatory said the central Syrian city was "completed surrounded" by the army, and that "no one has the right to enter or leave."

Regime troops closed off several roads around Baba Amr, which was almost entirely under siege.

Earlier, activists said the rebels had entered the district by stealth before launching their attack.

"The rebels infiltrated Baba Amr during the night. Those manning the army checkpoints barely had time to realise what was going on," said Omar, an activist who was also in touch with the insurgents.

Baba Amr gained notoriety during last year's bloody siege, with dozens of bodies also found in neighbouring districts of Homs, including those of people fleeing the fighting, which also claimed the lives of two foreign journalists.

American reporter Marie Colvin of The Sunday Times in Britain and French photojournalist Remi Ochlik were among those who died when a makeshift media centre in Baba Amr was shelled by Syrian forces.

The army, which controls around 80 percent of Homs, launched an offensive several days ago aimed at capturing rebel enclaves, notably in the northern Khaldiyeh district and in the old city, using helicopters to bombard them.

In a video posted on the Internet on Sunday, a rebel announced "a 'great victory battle' to liberate neighbourhoods (controlled by the army), namely Baba Amr, and ease the pressure on our comrades and on besieged Homs districts."