Air strikes by Syrian government forces have targeted rebel-held police stations inside Aleppo city, as visiting peace envoy Lakhdar Brahimi met with opposition groups in the capital Damascus.
An AFP correspondent also said three loud explosions were heard in the capital Damascus, where joint UN-Arab League envoy Brahimi spent his first night ahead of meetings with the opposition parties tolerated by President Bashar al-Assad's government.
Hassan Abdul-Azeem, a spokesman for Syria's opposition National Co-ordination Committee for Democratic Change, said a delegation from the group had met with Brahimi on Friday to discuss how to resolve the crisis.
"We support Mr Brahimi... and we will co-operate with him because the violence has reached [unprecedented] levels and the Syrian people are suffering from the killings, destruction and displacement," he said.
Abdul-Azeem's bloc, which is tolerated by the Syrian government, includes Arab nationalists, Kurds and socialists.
"We came to Syria for meetings with our Syrian brothers because there is a big crisis. I think it is getting worse," Brahimi, a veteran Algerian diplomat, was quoted as saying by Syria's official SANA news agency.
The envoy was expected to meet President Assad on Saturday.
Brahimi, who was appointed earlier this month, arrived at the Damascus airport on Thursday when at least 125 people died in violence across Syria, according to a watchdog.
Brahimi, who succeeded former UN chief Kofi Annan following the failure of his six-point peace plan, met Foreign Minister Walid Muallem on Thursday night.
Muallem assured Brahimi of "Syria's full co-operation" and stressed that any initiative must be based on "the interests of the Syrian people and their freedom of choice without foreign intervention," SANA reported.
On the battlefront, Syrian regime forces on Friday used fighter jets and helicopter gunships to pound the city and province of Aleppo, where fierce clashes raged around a military airport, monitors said.
In-depth coverage of escalating violence across Syria
Warplanes bombarded the rebel-held towns of Al-Bab and Marea near Aleppo city, said the Syrian Observatory for Human Rights, adding that army forces and rebels fought around Minnigh military airport.
Despite shelling by regime forces, as seen in videos posted online, residents of Marea, Aleppo city and towns across the northern province came out for anti-regime demonstrations after the weekly Muslim prayers on Friday, activists said.
Demonstrations were also reported in the provinces of Damascus, Idlib, Daraa in the south and Hama in central Syria where an unknown number of protesters were killed as regime forces opened fire in Hama city, the Observatory said.
Just south of Damascus, the army fired live ammunition at an anti-regime demonstration in the Palestinian refugee camp of Yarmouk, according to the Local Co-ordination Committees, a network of activists on the ground.
Near the capital, at least 15 soldiers were killed or wounded in an attack on their vehicle in the restive town of Douma, where clashes broke out near the municipal building, the Observatory said.
'Arms imports must stop'
In neighbouring Lebanon, the army seized a truck loaded with weapons believed to be heading to Syria. The cache included light arms, hand grenades, rockets, and communication devices, the army said.
Also in Lebanon, Pope Benedict XVI on Friday started a weekend visit with a call for an end to arms imports to Syria.
"Arms imports must stop once and for all, because without arms imports, war cannot continue," he told reporters on his plane.
Turkish Prime Minister Recep Tayyip Erdogan, meanwhile, said the Assad regime was nearing its "inevitable" end and that his government was holding continuous contacts with the Syrian opposition.
"Assad's regime is approaching its inevitable end," Erdogan said in comments translated into Russian at a news conference in the Ukrainian Black Sea resort of Yalta.
"We must say 'No' to this human drama and not allow flames to engulf the whole region, so that the transition process could move more quickly ahead," the Turkish leader said.