Syrian army troops backed by pro-government militia members have entered rebel-held neighbourhoods of the central city of Homs after laying siege to the districts for nearly two years.

Activists on the ground and the Britain-based Syrian Observatory for Human Rights monitoring group on Tuesday both confirmed the operation.

"The Syrian army and the National Defence Forces have achieved key successes in the Old City of Homs," Syrian state television said.

It said troops were advancing in several besieged neighbourhoods in the area, and had "killed a number of terrorists".

"They have entered into one area, Wadi al-Sayeh, which lies between Juret al-Shiyah and the Old City," said Abu Bilal, an activist trapped inside the blockade, who spoke to AFP news agency via the internet.

Rami Abdel Rahman, director of the Syrian Observatory, said regime forces began the advance a day earlier.

"The military operation began yesterday after National Defence Forces were deployed to strengthen the regime troops' presence," he told AFP.

Elsewhere in Syria, two mortar rounds landed near schools in predominantly Christian districts of Damascus, killing one child and wounding 41 other people, state media said.

Syria's official news agency said one of the shells struck a school in the Bab Touma neighbourhood, killing one child and wounding 36 others.

In a separate attack, another mortar round exploded near the Mar Elias Church in the Dweilaa district, wounding five people.

On Tuesday, another town in the area, Assal al-Ward, fell into government hands, state TV said.

As the fighting continued, the Al-Watan daily said that the speaker of parliament would announce the date of the country's presidential elections next week.

They are expected to be held in June, before the end of Assad's seven-year term on July 17.

Electoral rules require candidates to have spent the last 10 years in Syria, effectively preventing the opposition-in-exile from competing against him.

Saudi criticism

For his part, on Tuesday Prince Saud al-Faisal, foreign minister of Saudi Arabia, whose government strongly backs the Syrian opposition, joined international criticism of Syria's decision to press ahead with the vote.

"The announcement by the Syrian regime to hold elections is an escalation and undermines Arab and international efforts to peacefully resolve the crisis based on the Geneva I conference," Saud said in Riyadh.

Meanwhile, on ground a rebel source told AFP news agency they received at least 20 US-made TOW anti-tank missiles from a "Western source"

"Moderate, well-organised fighters from the Hazm movement have for the first time received more than 20 TOW anti-tank missiles from a Western source," the source said on condition of anonymity, and without specifying who had supplied the rockets.

The Hazm movement, part of the opposition Free Syrian Army, brings together mainly ex-army officers and soldiers who defected from the military to join the revolt.

"More have been promised should it be proven that the missiles are being used in an effective way," the source said.

"Dozens of fighters have been trained with international assistance in the use of these missiles."

The source said the weapons have been used in flashpoint areas of Idlib, Aleppo and Latakia provinces in the north.

Amateur video distributed by the opposition Masarat media network showed rebels unpacking, loading and firing several missiles at unnamed locations in the Syrian countryside.

"Most of the targets were tanks," said the rebel official, adding that "the 20 missiles have been used 100 percent effectively, always hitting their targets."