Syrian forces are reported to have fired tear gas and live ammunition into crowds of protesters in the northern Syrian city of Aleppo, in what opposition activists say is one of the biggest demonstrations since the anti-government uprising began 15-months-ago.

Reports say at least two people have been wounded in the violence, although details are unclear.

The protests come after Syrian security forces disrupted a student demonstration in Aleppo on Thursday. Video taken from a U.N. vehicle in Aleppo Thursday showed security forces beating student protesters.

Meanwhile, online video posted by opposition groups on Friday showed shells hitting parts of the rebel stronghold of Rastan, and others showed government troops shelling parts of nearby Homs.

The protests and the violence come as United Nations officials express concerns that foreign-based militants 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 pursue 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.

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.”

A group calling itself the “Nusrat Front” claimed responsibility earlier this week for the blasts. However, many analysts say claims of al-Qaida involvement have yet to be proven and may not be credible.

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.