Syrian forces bombarded rebel strongholds around Damascus Monday and rounded up residents in a massive raid in the heart of the capital as fighting raged in the second city of Aleppo, activists said.
Security forces raided houses and shops in the centre of Damascus in the biggest operation of its kind in the city since the launch of the uprising against President Bashar al-Assad's regime in March last year, they said.
Foreign ministers of Muslim states are due to hold talks in Saudi Arabia on Monday as international concern mounts over the increasingly vicious conflict that has triggered a major humanitarian crisis and sent hundreds of thousands of Syrians fleeing.
The UN Security Council is also due to debate the future of the UN mission in Syria on Thursday, but so far there is little consensus among world powers on how to deal with the conflict, with China and Russia vehemently opposing all resolutions against the Assad regime.
As shells slammed into rebel strongholds in the province of Damascus from before dawn Monday, security forces raided the Old City and several other districts of the capital, according to the Syrian Observatory for Human Rights.
"This is not the first time there have been raids in central Damascus, but this is the first time something like this has happened on such a large scale," Observatory director Rami Abdel Rahman told AFP.
The Observatory said 21 people had been arrested and that security forces also swept into a graveyard "under the pretext of searching for weapons".
The Syrian Revolution General Council, a network of activists on the ground, said that during the raids armed forces had broken down the doors of shops closed in a show of defiance against the regime.
The Britain-based Observatory said more than 45 people including 36 civilians had been killed in the past 48 hours in Damascus province alone.
About 146 people were killed on Sunday in fighting across Syria, including the Damascus area, the commercial hub of Aleppo and in Syria's third city of Homs.
Fighting also raged in the city of Aleppo, whose fate is seen as potential turning point in the conflict as rebels seek to establish a strategic foothold in the north.
Syrian troops were advancing on the southwestern rebel stronghold of Sukari, security sources in Damascus said.
"Opposition fighters carried out a dawn attack on a key air force intelligence branch in Aleppo's Zahraa district in the west of Aleppo," Abdel Rahman said.
Fighting also broke out in the southwestern district of Salaheddin, which rebels fled last week but has seen continued clashes since, he said.
Salaheddin "is controlled by regime forces, but there are pockets of resistance," Abdel Rahman added.
The Observatory said Sunday that "communications of all forms have been cut off" in Aleppo itself, as well as large areas of the province.
The opposition Syrian National Council and activist groups on Sunday charged that pro-government militia had summarily executed 10 civilians during a round-up in the Shamas neighbourhood of Homs.
The Observatory confirmed that pro-government forces carried out a round-up in the district, but made no mention of any executions.
Pro-regime daily Al-Watan said military forces had "cleared the Shamas neighbourhood in Homs of terrorists" in an operation that lasted 48 hours, adding that "more than 40 insurgents were killed" while 70 others arrested.
Both sides have been accused of human rights violations in the escalating conflict whose outcome will have major repercussions for Syria's neighbours and the military and geopolitical balance of power in the Middle East.
More than 21,000 people have been killed across Syria since Assad's regime began a brutal crackdown on dissent 17 months ago, according to the Observatory. The toll is impossible to verify, and the United Nations has stopped maintaining an independent count.
As the conflict escalates, the international community is still grappling with how to tackle the crisis.
Foreign ministers of Muslim states were due to meet later Monday in the city of Jeddah ahead of an Islamic summit hosted by regional powerhouse Saudi Arabia on Tuesday to drum up support for the anti-regime revolt.
US Secretary of State Hillary Clinton held talks in Turkey at the weekend, saying their "number one goal" was to hasten the end of Assad's regime and stop the bloodshed, amid talk of setting up humanitarian corridors or no-fly zones.
The United States slappped a new round of sanctions on the Assad regime on Saturday and added the Syrian-backed Lebanese militia group Hezbollah to a blacklist of organisations targeted under the sanctions.
Meanwhile, diplomats in New York say veteran Algerian diplomat Lakhdar Brahimi, a former top Arab League official, has been tipped to replace Kofi Annan as the international Syria envoy.