Syrian troops backed by tanks and warplanes on Wednesday fought to repel an attack on the central prison in Aleppo after rebels blew up its walls in suicide car bombings, a watchdog has said.

"Fierce fighting is taking place within the walls of the compound," of the prison in the northern city, said the Syrian Observatory for Human Rights, voicing concern over the fate of the inmates.

About 4,000 prisoners, including Islamists and common law criminals, are held in the prison on the outskirts of the city, which is largely under rebel control, said Observatory head Rami Abdel Rahman.

Regime forces fired tank shells and launched air raids around the jail to repel the rebels, igniting fires and damaging neighbouring houses, the Observatory said.

A child was killed and seven members of a family were wounded in the bombings, it added, citing activists on the ground.

In the capital, an explosion rocked Ummayad Square in central Damascus, an AFP journalist said.

The Observatory said one person was killed in the blast, which a soldier at the scene said was caused by a sticky bomb attached to a car near a checkpoint outside the Army Chief of Staff Offices.

Elsewhere in the capital, at least four people were killed and 12 others injured in regime shelling on the area of the Palestinian Yarmuk camp, the group added.

The Observatory, which relies on a network of activists and medics across the country, also reported clashes in the central province of Hama and on the international highway in Damascus province.

Elsewhere, violent clashes were reported in Idlib province of northwestern Syria and in Daraa in the south.

In central Homs, the group said, two rebel fighters were killed in ongoing battles with regime forces backed by members of Lebanon's militant group Hizbullah.

The official news agency SANA, meanwhile, said Internet and telephone lines were down because of a faulty fibre optic cable, in the second such incident in the past week.

Syria's Internet was down for two days last week, with state media blaming the blackout on a technical fault, but with activists and a watchdog accusing the regime of deliberating cutting the connection to shield military operations.

In violence across the country Tuesday, 99 people were killed, said the Observatory, which has compiled a total of more than 94,000 deaths in the two-year conflict between the Bashar Al-Assad regime and its opponents.

It said an initial toll for Wednesday included at least 38 people killed.