The UN Secretary General Ban Ki-moon says that Islamist militants from al-Qaeda must be behind two deadly suicide car bomb attacks in Syria last week.
Fifty-five people were killed and 372 were wounded when two car bombers blew themselves up in the capital Damascus on 10 May.
It was the deadliest attack on the city since the uprising against President Bashar al-Assad started last year.
Mr Ban also said the death toll from the crisis now stands at 10,000 people.
"Very alarmingly and surprisingly, 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," Mr Ban said on Thursday.
Syrian officials have blamed "foreign terrorists" for the twin bombings.
Earlier this month Syria sent a list of 26 names to the United Nations of foreign nationals it had apprehended, claiming most of them were members of al-Qaeda.
Aleppo university rally
Activists have called for Friday protests in support of students who held a large rally at Aleppo University on Thursday.
They were demanding the removal of Mr Assad and his government during a visit from UN observers.
The protest was streamed live on the internet from mobile phones using the Bambuser website.
Footage shows scores of demonstrators chanting loudly. A YouTube clip apparently from the same rally shows students climbing on top of a UN vehicle.
A voice can be heard in the background saying in English "the University of the revolution".
Another YouTube video, filmed through the windscreen of a car, appears to show a demonstrator being beaten by a group of men wielding sticks.
Two-hundred and fifty-seven unarmed UN observers have been deployed in Syria to monitor a ceasefire which has largely collapsed since it was brokered in March by UN and Arab League representative Kofi Anan.
Mr Ban said their deployment had some "dampening effect" on the violence, but not enough to halt it.
"We are trying our best efforts to protect the civilian population," Mr Ban added.