Syrian air strikes in the central city of Homs on Sunday killed three civilians as regime forces pressed forward with a new bid to retake several rebel-controlled districts, a watchdog said.

The Syrian Observatory for Human Rights added that at least 24 members of the regime forces had also been killed in the fighting in Homs that began on Saturday.

"Syrian warplanes carried out air strikes on the Old City of Homs... destroying a house and causing three deaths," the Syrian Observatory for Human Rights said.

"Regime forces also carried out heavy shelling of the districts of Khaldiyeh and the Old City, and the sound of explosions could be heard," the group added.

Observatory director Rami Abdel Rahman said at least three civilians, a woman and two children, were killed in the air strikes in the Old City, and that dozens of people had been injured since the army assault began.

"The army is continuing its attempt to enter Khaldiyeh, but it hasn't succeeded so far," he said.

The Observatory called the bombardment of several parts of Homs, including Khaldiyeh, Bab Hud, Hamidiyeh and Bustan al-Diwan, "unprecedented".

A security source in Damascus said on Saturday regime forces wanted to "cleanse those neighbourhoods of Homs that are in the hands of armed terrorists, particularly Khaldiyeh, Hamidiyeh and the Old City".

Syria's Al Watan daily, which is close to the government, said on Sunday that the army had "made qualitative new advances in the city of Homs amid fierce clashes with armed militias in Khaldiyeh and Bab Hud".

And state news agency SANA cited a military source as saying "a number of terrorists were eliminated and their weapons and ammunition destroyed in Khaldiyeh neighbourhood".

The operation prompted Syria's key opposition National Coalition to issue a statement late on Saturday urging "battalions of the Free Syrian Army to come to the aid of Homs with all means possible".

The group also called on international backers of the uprising to establish a no-fly zone and launch air strikes against regime military bases.

Homs, the third largest city in the country, was one of the first to join the revolt against President Bashar al-Assad's regime more than two years ago.

Khaldiyeh and the Old City have been under siege by the army for around a year.

Elsewhere, the Observatory said at least six people were killed when rebels in Aleppo province shot down a helicopter.

SANA confirmed that a government helicopter was "targeted by terrorists over the town of Nabl".

The state news agency said education officials carrying official examination papers had been on board.

"The targeting led to the helicopter crashing and the death of seven officials and the helicopter crew," the agency said, without specifying the number killed.

Prime Minister Wael al-Halqi, quoted by Syrian state television, called the attack "cowardly terrorist act against... education workers seeking to spread education and knowledge".

The Observatory, which relies on a network of activists, doctors and lawyers on the ground, said at least 111 people were killed throughout the country on Saturday -- 36 civilians, 35 rebels and 40 regime forces.