Syrian rebels began a coordinated assault on the main prison in the northern city of Aleppo today in an attempt to free hundreds of regime opponents believed to be held in the facility.

Aleppo emerged as one of the major fronts in the country's civil war after a rebel offensive there in July, and the fighting since then has settled into a bloody stalemate.

The city, Syria's largest, holds strategic and symbolic value, and both sides have taken significant losses in the battle to expand the areas under their control.

The Britain-based Syrian Observatory for Human Rights said rebels detonated two car bombs simultaneously outside the walls of the central prison this morning before trying to storm the facility.

Fierce clashes are taking place between President Bashar Assad's troops and opposition fighters around the detention centre, according to Observatory director Rami Abdul-Rahman.

There were no immediate reports of casualties.

The city's central prison is believed to be holding 4,000 prisoners, around 250 of whom are jailed for reasons related to the 26-month-old uprising against Mr Assad's regime, said Abdul-Rahman, who relies on reports from a network of informants based in Syria.

For weeks, rebel fighters have been battling government troops in the area around the prison to try to seize the facility and free the prisoners.

Earlier this month, the rebels overran the headquarters of the government's anti-terrorism forces, located near the jail.

Meanwhile, an internet blackout has engulfed the country for the second time in two weeks.

The outage appeared similar to last week's blackout, Syrian residents and the US-based Renesys Corp said.

"It looks like a replay of what happened on the seventh and eighth," Renesys chief technology officer James Cowie said by telephone, referring to last week's nationwide outage.

He said the cause was not immediately clear.

"It's entirely consistent with a technical fault at a central facility; it's also completely consistent with a decision to use an internet kill switch," he said.

Preliminary data from Google Inc's Transparency Report website also pointed to a nationwide blackout, with Syria's online traffic share falling to 0% this morning.

Syrian government websites, including the SANA state news agency, appeared to be down, but SANA reported on its twitter account a technical problem.

It said maintenance units were working to restore the internet as soon as possible. It did not elaborate.

An official at the Syrian communications department said an internet cable was cut in a Damascus suburb and that it will take up to four hours to fix.

The official spoke on condition of anonymity because he was not authorized to give official statements. He did not say what caused the cut.

Syrian authorities have shut off phone and internet service in select areas in the past to disrupt rebel communications when regime forces were conducting major operations.

Such widespread blackouts, however, have been rare, and the reason for the 7 May outage is still unclear.