(CNN) -- Tensions boiled in Syria's two largest cities Friday as violence erupted in Aleppo and the U.N. secretary-general sought to determine whether al Qaeda was involved in last week's suicide attack in Damascus.
Security forces opened fire on anti-government protesters as they attempted to break up a demonstration that began after dawn prayers in Aleppo, according to the Local Coordination Committees of Syria, an opposition network that counts casualties and organizes anti-government protests.
A bomb apparently targeted security forces in Aleppo early Friday, killing one soldier and wounding five others, according to Rami Abdul-Rahman of the London-based Syrian Observatory for Human Rights.
The city, the most populous in Syria, is widely considered a stronghold of President Bashar al-Assad.
Although there have been a number of demonstrations in Aleppo since the Syrian uprising began, the number of people publicly calling for al-Assad's ouster has increased significantly. In an outpouring Thursday, thousands purportedly took to the streets, calling for an end to the regime.
The increase in demonstrations in Aleppo comes at a critical time for both sides, which are vying for international support in the conflict that began in March 2011 with protests calling for political reforms. It quickly devolved into an armed uprising after a brutal crackdown by al-Assad's forces.
Opposition groups, including members of the rebel army, say al-Assad's government has been trying to thwart their efforts by accusing them falsely of links to terrorists. Al-Assad has dismissed the opposition as composed of foreign-backed terrorists bent on destabilizing the government.
A string of attacks has erupted in recent months in the power centers of Damascus and Aleppo.
The government said it foiled a terrorist suicide strike in Aleppo on May 11. That occurred a day after twin suicide bombings in the capital killed at least 55 people, wounded a few hundred others and caused widespread damage, also blamed on terrorists.
The Damascus strike was the deadliest attack in Syria since the unrest began.
Such incidents have heightened concerns about the possibility of jihadists operating in the country. One group, called the Al Nusrah Front for the People of the Levant, has claimed responsibility for suicide attacks in Syria.
A video purportedly from Al Nusrah Front released Saturday said government buildings in Damascus were targeted "because the regime continues to shell residential civilians." But a statement this week from the group said the video "is full of fabrications."
"We never received any confirmation, acknowledgment or denial from our military sector in the front," the group said.
U.N. Secretary-General Ban Ki-moon, speaking Thursday at the United Nations, blamed al Qaeda for the Damascus attack.
"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," he said.
But on Friday, he said in New York, "We are still trying to investigate who are these third forces behind this terrorist attack in Damascus."
CNN cannot independently verify reports of deaths and violence because the Syrian government has severely restricted access by international media.
The reports of violence have cast severe doubts on the success of the peace plan brokered by Annan.
Ban and Mood said that 260 observers of 300 planned were deployed in Syria to monitor the progress of the peace plan, which calls for a cease-fire.
"The deployment of monitors has some dampening effect," Ban said. "The number of (instances of) violence has reduced, but not enough. Not all the violence has stopped. So we will continue, as much as we can, to protect the civilian population."
Mood said officials have been "very pleased to see and witness an immediate calming effect brought about by our arrival," but "this has been challenged by asymmetric incidents, sometimes intense, but in some locations."
Ahmad Fawzi, spokesman for U.N.-Arab League joint special envoy Kofi Annan, said "both sides" have violated the cease-fire.
The opposition took to social media to urge Syrians across the country Friday to take to the streets in solidarity with the Aleppo demonstrators.
Regime security forces also opened fire at anti-government protesters in the cities of Daraa, Jasem and Boukamal, the Local Coordination Committees of Syria said. There were also reports of security forces shelling the beleaguered opposition stronghold of Rastan in Homs province, the Local Coordination Committees said. The Syrian Observatory for Human Rights reported countrywide mass protests demanding the downfall of the regime.
At least 27 people died in Syria on Friday, the Local Coordination Committees said. The deaths included three women and three children and occurred in the provinces of Homs, Hama, Idlib, Daraa, Aleppo, Damascus and the Damascus countryside.
Meanwhile, Syria's Central Judicial Committee is investigating the attacks against the U.N. observers and will send "as soon as possible the results of its investigations to the Central Committee," the state-run Syrian Arab News Agency reported Friday.
A video posted Thursday on YouTube purported to show thousands of anti-government protesters outside Aleppo University, where men in blue helmets could be seen. The U.N. observers wear blue helmets.
Meanwhile, another video surfaced on YouTube that purports to show Lebanon's military arresting a Syrian activist at a hospital in Tripoli where he was being treated for wounds he suffered when Lebanese forces attempted to break up clashes between pro- and anti-Syrian regime groups.
CNN can not independently confirm the authenticity of the videos.
In recent days, violence has spilled into Lebanon and Turkey, where thousands of Syrians have fled. At least one person was killed and an undetermined number were wounded Thursday in renewed clashes in the northern Lebanese city of Tripoli between factions supporting and opposing the uprising in Syria, Lebanon's National News Agency reported.
Ban says that 9,000 to 10,000 people have been killed during the crisis. Activists have said the death toll is higher.
CNN's Amir Ahmed, Hamdi Alkhshali and Richard Roth contributed to this report.