Thousands of protesters are taking to the streets in the northern Syrian city of Aleppo in what opposition activists are saying is one of the biggest demonstrations since the anti-government uprising began 15-months-ago.

Activists have been urging protesters to rally behind a slogan of “heroes” of Aleppo University in a show of support for students who were confronted by security forces during a protest on Thursday.

The protests and the violence come as United Nations officials express concerns that al-Qaida may have played a role in recent attacks that have further strained a fragile cease-fire.

Hopes for peace may get some boost from international envoy Kofi Annan. A spokesman says the former U.N. chief is planning to return to Damascus to again work on finding a peaceful solution to the conflict. But the spokesman said there is not a timetable yet for what would be Annan's first visit since brokering a cease-fire agreement in March.

Meanwhile, U.N. Secretary-General Ban Ki-moon says he believes al-Qaida is behind twin bombings in Damascus, last week, that killed 55 people. He says such involvement would create “very serious problems.”

In a Friday news conference, the U.N. observer mission head, Major General Robert Mood, said an al-Qaida presence would be a “worrying development.”

“I'm concerned about the incidents where explosives, improvised devices, are targeting innocent civilians, innocent people, because it is not going to help the situation.”

Mood said the number of U.N. monitors in Syria had grown to 260, but monitors alone could not stop the deadly violence.

“No volume of observers can achieve a progressive drop and a permanent end to the violence if the commitment to give dialogue a chance is not genuine.”

Also Friday, opposition activists said there was intense government shelling in Rastan, an opposition stronghold in the Homs region. On Thursday, the Syrian Observatory for Human Rights said machine gun fire killed three people in the region.

The observatory and Syrian state-run media say assailants opened fire on a bus carrying law enforcement officers in Homs on Thursday, killing one person. The government blamed “armed terrorists” for the attack.

The United Nations says the death toll from violence related to the anti-government uprising that erupted more than a year ago may now reach 10,000.