Syrian warplanes struck rebel enclaves in flashpoints across the country on Wednesday, while a raid in the northern city of Raqa killed and wounded dozens of people, a watchdog said.

Fighter jets also bombarded Homs in the centre, on the fourth day of a major offensive aimed at crushing the insurgency in the country's third-largest city, said the Syrian Observatory for Human Rights.

Air strikes on Raqa came two days after rebels overran most of the strategic provincial capital in their biggest victory since the revolt against President Bashar al-Assad's rule erupted in March 2011, the Observatory said.

"Dozens of people were killed or wounded in air strikes in Raqa city that targeted the areas around security and government buildings" that the rebels seized during their advance, said the Britain-based watchdog, without being able to provide exact figures.

While insurgents were in control of much of the city, the area around the military intelligence building remained under regime control and clashes were continuing there, said the Observatory.

Raqa has "for two days been the theatre of an invasion of terrorists and criminals", said pro-regime daily Al-Watan, echoing the official term of "terrorists" to describe anti-Assad rebels.

But Information Minister Omran al-Zohbi said "there is nothing to worry about regarding the presence of armed groups in parts of Raqa. It's just a question of time before (the army) recaptures it", state news agency SANA quoted him as saying.

Meanwhile, warplanes raided the city of Homs, dubbed "capital of the revolution" by activists.

In fierce battles on the edges of rebel-held districts, insurgents tried to fight off the army and pro-regime militia's advance, said the Observatory, adding that "troops pounded the Old City and Khaldiyeh districts" of the city.

Earlier, the Observatory had reported the army's use of helicopters, warplanes and rocket fire to strike Khaldiyeh.

Although the army now controls some 80 percent of Homs, several districts remain under rebel control despite a suffocating eight-month siege and several hundred civilians are trapped there.

"We don't know how they can get out, or where they would even go, should the army seize control of the rebel districts. Homs is surrounded," Observatory director Rami Abdel Rahman said.

Near Damascus, the air force bombarded several rebel enclaves in the Eastern Ghouta area, among them the battered town of Douma, a stronghold of the rebel Free Syrian Army, said the watchdog.

And in the northwest, warplanes bombarded Hafsarja and Deir Sharqi in Idlib province, much of which is in rebel hands, said the Observatory.

The United Nations says at least 70,000 people have been killed since the start of the uprising.