BEIRUT -- Syrian warplanes and artillery hit targets near Damascus International Airport on Friday following a particularly bloody day of attacks in the capital that killed dozens and struck deep into the heart of the city.
There were no immediate reports of casualties from Friday's shelling, which targeted the towns of Beit Sahm and Shebaa near the main airport road south of the capital, activists said.
The Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees also reported clashes in the rebel strongholds of Daraya and Moadamiyeh, southwest of Damascus.
Recent rebel advances in the Damascus suburbs, combined with the bombings and three straight days of mortar attacks, mark the most sustained challenge of the civil war for control of Assad's seat of power.
Syrian state media said the car bombing on Thursday in the heart of Damascus -- near the ruling Baath Party headquarters and the Russian Embassy -- was a suicide attack that killed 53 civilians and wounded more than 200, including children. Anti-regime activists put the death toll at 61, which would make it the deadliest Damascus bombing of the revolt. The different tolls could not be reconciled because the regime restricts independent media access.
The main opposition group, the Syrian National Coalition, condemned the attack without accusing any specific group of carrying it out. It did, however, suggest that the regime allowed foreign terror groups to operate in Syria.
"The terrorist Assad regime bears the most responsibility for all the crimes that happen in the homeland because it has opened the doors to those with different agendas to enter Syria and harm its stability so it can hide behind this and use it as an excuse to justify its crimes," the group said in a statement.
There was no immediate claim of responsibility for the attack, but suspicion will likely fall upon one of the most extreme of Syria's myriad rebel factions, the al-Qaida-linked Jabhat al-Nusra. The group, which the U.S. has designated as a terrorist organization, has claimed similar past bombings against regime targets.
On Friday, the Coalition said it would welcome U.S. and Russian mediation to negotiate a peace deal to end the country's civil wall but insisted it would not allow Assad or members of his security services to participate in the talks.
The announced came in a statement posted on the Coalition's Facebook page following two-day meetings in Cairo meant to try to firm up its position on whether to engage with the regime in talks.
"Bashar Assad and the security and military leadership responsible for the state of Syria today must step down and be considered outside this political process," the statement said. "They cannot be part of any political solution for Syria and must be held accountable for their crimes."
SNC chief Mouaz al-Khatib has angered some of his colleagues by offering talks with regime elements to help end the civil war. Friday's announcement appeared meant to set the boundaries for any future talks by stressing that Assad and his aides cannot be part of any negotiations.
The violence in Damascus follows a string of tactical victories in recent weeks for the rebels -- the capture of the nation's largest hydroelectric dam and the overtaking of air bases in the northeast -- that have contributed to the sense that the opposition may be gaining momentum.
But Damascus is the ultimate prize in the civil war, and many view the battle for the ancient city as the most probable endgame of a conflict which has killed nearly 70,000 people, according to U.N. estimates.
The latest violence in the capital has shattered the sense of normalcy that the Syrian regime has desperately tried to maintain in Damascus, a city that has largely been insulated from the bloodshed and destruction that has left other urban centers in ruins.
The rebels first launched an offensive on Damascus in July, following a stunning bombing on a high-level government crisis meeting that killed four top regime officials, including Assad's brother-in-law and the defense minister. After that attack, rebel groups that had established footholds in the suburbs pushed in, battling government forces for more than a week before being routed and swept out.
Since then, government warplanes have pounded opposition strongholds on Damascus' outskirts, and rebels have managed only small incursions on the city's southern and eastern sides.
Also Friday, New York-based Human Rights Watch urged the U.N. to require Syrian authorities to grant international monitors access to its detention facilities, following the death of a peace activist in custody.
Omar Aziz, 64, died on Feb. 16 of health complications at a military hospital, the group said in a statement. It went on to describe how a newly released detainee also reported witnessing the death of Ayham Ghazzoul, an imprisoned 26-year-old rights activist. Both had been detained by security forces in November.
"Aziz's death, and Ghazzoul's feared death are yet another reminder of the need to immediately lift the veil of secrecy over Syria's prisons," said Sarah Leah Whitson, HRW's Middle East director. "How many more deaths in custody before the Security Council requires Syria to open up its detention centers?"
Human rights groups and the opposition accuse Syrian authorities of holding tens of thousands of prisoners, many of whom it is feared have been being tortured.