UN Secretary-General Ban Ki-moon said Thursday he believed al-Qaeda was behind the twin suicide car bomb blasts that killed at least 55 people in Syria last week.

A few days ago there was a huge, serious, massive terrorist attack. I believe that there must be al-Qaeda behind it. This has created again very serious problems, Ban told a youth event at UN headquarters in New York, according to a Reuters report.

He said about 10,000 people have died in the country's 14-month long uprising against President Bashar al-Assad's regime.

The twin suicide car bombings outside a military intelligence building in Damascus on May 10, which killed 55 and injured 372, were the deadliest attacks against a government target in the 14 months of bloody fighting.

The Syrian regime has maintained that it is facing a terrorist conspiracy directly supported and funded by foreign establishments.

Earlier this month, Syria named 26 foreign nationals, who had been apprehended in the country for aiding Syrian rebels. In a report sent to the UN, the Assad regime described 20 of those as members of al-Qaeda who had gained entry into the country from Turkey, a Reuters report said.

US President Barack Obama had earlier condemned the attacks and expressed concern that al-Qaeda might be increasingly taking advantage of the extended periods of the political mayhem.

Defense Secretary Leon Panetta told reporters last week that US intelligence indicated an al-Qaida presence in Syria, but said the extent of its activity remained unclear.

Frankly we need to continue to do everything we can to determine what kind of influence they're trying to exert there, Panetta said.

There are 257 unarmed UN observers in Syria as part of the team, implementing the five-week-old ceasefire brokered by the UN-Arab league peace envoy Kofi Annan.

Syrian forces opened fire on rebels in Aleppo Friday, the opposition said, in an effort to dissolve the demonstration that had begun a day before. There has been a significant increase in the number and size of demonstrations in Aleppo, signaling a critical juncture for both sides that are vying for international support.

However, Ban said the deployment of monitors had some dampening effect.

The deployment of monitors has some dampening effect, the number of violence has reduced but not enough, not all the violence have stopped, Ban said. We are trying out best efforts to protect the civilian population.