Grisly footage of apparent atrocities in Syria triggered outrage Monday, as regime forces bombarded rebel strongholds around Damascus and launched a mass raid in the historic heart of the capital.

The graphic videos posted on YouTube showed what appeared to be rebels callously throwing bodies off a post office building in a city near the northern metropolis of Aleppo, while another showed a man, blindfolded and bound, as his throat was savagely cut.

Fighting was also raging in the northern metropolis of Aleppo, where security forces were advancing on an opposition-held district but where all communications have reportedly been cut.

With the international community deadlocked over how to end 17 months of bloodshed, the opposition umbrella group the Syrian National Council appealed for the establishment of no-fly zones.

And in a new blow for embattled President Bashar al-Assad, Syria's top representative at the UN Human Rights Council said he has defected, the latest in a line of senior officials to flee the regime.

International concern is mounting over how to end a conflict that has triggered a major humanitarian crisis and sent hundreds of thousands of Syrians fleeing, with at least 100 people being killed daily.

Foreign ministers of Muslim states hold talks in Saudi Arabia Monday while the UN Security Council -- which has so far failed to reach a consensus on how to stop the bloodshed -- meets on Thursday to debate the future of its mission.

In one shocking amateur video posted Monday, several bodies were seen crumpled on the ground outside a post office building in Al-Bab city before another three are hurled from the rooftop as the crowd cries "This is a shabiha," referring to the pro-government militia.

In another, a group of men forced a blindfolded man, with his hands tied behind his back, down to the ground in Aleppo while an assailant forced what appeared to be a small knife repeatedly across his throat as his blood spurted.

"If these videos are confirmed, such atrocities harm the revolution. They only benefit the regime and the enemies of the revolution," Syrian Observatory for Human Rights director Rami Abdel Rahman told AFP.

Both sides in the increasingly vicious conflict have been accused of human rights violations as reports of cold-blooded killings mount, although the authenticity of the latest videos could not be verified.

Also Monday, security forces arrested residents in a major operation in the heart of Damascus, including the historic Old City, while shells slammed into rebel strongholds around the capital from before dawn, according to the Britain-based Observatory.

It was biggest operation of its kind in the city since the launch of the uprising against Assad, the Observatory said.

It said 21 people had been arrested and that security forces also swept into a graveyard "under the pretext of searching for weapons", while other activists said the troops had broken down the doors of shops closed in a show of defiance against the regime.

The Observatory said 50 people had been killed on Monday, including 28 civilians in violence across the country.

In Aleppo Monday, government troops were advancing on the southwestern rebel stronghold of Sukari, security sources in Damascus said. The Observatory meanwhile said opposition fighters attacked a key air force intelligence branch in the western Zahraa district.

Fighting also broke out in the southwestern district of Salaheddin, which rebels fled last week but has seen continued clashes since, it said.

The fate of Aleppo -- Syria's largest city -- is seen as potential turning point in the conflict whose outcome will have major repercussions for Syria's neighbours and the military and geopolitical balance of power in the region

More than 21,000 people have been killed across Syria since Assad's regime launched its brutal crackdown on dissent, with fighting escalating after the failure of former envoy Kofi Annan's peace plan.

Abdel Basset Sayda, who heads the opposition Syrian National Council, told AFP that the rebels wanted "two no-fly zones, one in the north, close to the Turkish border, and another in the south, close to the border with Jordan," in addition to "safe places for refugees and humanitarian corridors."

US Secretary of State Hillary Clinton held talks in Turkey at the weekend, after Washington imposed a new round of sanctions on Syria, saying their "number one goal" was to hasten the end of Assad's regime.

Foreign ministers of Muslim states were meeting Monday in Jeddah ahead of an Islamic summit Tuesday hosted by regional powerhouse Saudi Arabia on the Syria crisis.

UN humanitarian chief Valerie Amos said she will visit Syria and Lebanon from Tuesday.