More than 60 people were killed in a regime air strike on a bakery in a rebel-held town on Sunday, monitors said, as peace envoy Lakhdar Brahimi launched a new bid to resolve Syria's brutal 21-month conflict.

In one of the deadliest incidents of the conflict, the Syrian Observatory for Human Rights said the strike hit a bakery in Halfaya in the central province of Hama, killing more than 60 people and wounding at least 50.

Many of the wounded were in critical condition, it said.

"In Halfaya, regime forces bombarded a bakery and committed a massacre that killed dozens of people, including women and children, and wounded many others," said the Local Coordination Committees, a grassroots network of activists.

"A MiG (jet) has attacked! Look at (President Bashar) Assad's weapons.

Look, world, look at the Halfaya massacre," says an unidentified cameraman shooting an amateur video distributed by the Observatory.

The footage showed a bombed one-storey block and a crater in the road.

Bloodied bodies lay on the road, while others could be seen in the rubble.

Men carried victims out on their backs, among them at least one woman, the video showed.

On Monday, rebels launched an all-out assault on army positions across Hama, which is home to strong anti-regime sentiment.

During the summer, rights groups accused government forces of committing war crimes by dropping bombs and using artillery on or near several bakeries in the northern province of Aleppo.

Another of the bloodiest attacks in the Syrian conflict was on a bread line in the Qadi Askar district of Aleppo city on August 16 that left 60 people dead, according to local hospital records.

International envoy Brahimi, meanwhile, travelled to Damascus overland from neighbouring Lebanon on a previously unannounced visit, officials said.

The U.N.-Arab League envoy last visited on October 19, but since then there has been fighting between government forces and rebels on the road to Damascus airport.

During his October visit he met Assad and other officials to clinch a temporary ceasefire for the Muslim feast of Eid al-Adha. Despite pledges, the truce did not hold.

At least 44,000 people have been killed in violence across Syria since the outbreak of the anti-regime revolt in March 2011, according to the Britain-based Syrian Observatory.

Shortly before Sunday's air strike, Information Minister Omran al-Zohbi reiterated calls for national dialogue. "We tell those who do not want dialogue to engage in talks, because time is running out," he told reporters.

The minister also played down Vice President Faruq al-Sharaa's assessment that the conflict could not be resolved by military means.

"It is one opinion among 23 million opinions in Syria, which is a state led by institutions and leaders who will give the final opinion," said Zohbi.

Sharaa said in a Lebanese newspaper interview published last Monday that a clear winner was unlikely to emerge in Syria's war and he preferred a negotiated solution, in remarks at odds with Assad.

Including those killed in Halfaya, at least 174 people -- 108 of them civilians -- were killed on Sunday across Syria, said the Observatory.

Other air strikes included a raid in Aleppo province. "At least 13 people were killed in an air raid on the town of Sfeira," said the group, which relies on a network of doctors, activists and lawyers for its information.

A Palestine Liberation Organization official, meanwhile, told a conference in Cairo that more than 700 Palestinians have been killed in the conflict, including in the Yarmuk refugee camp of southern Damascus.

"We have asked the Syrian authorities not to let Palestinians be drawn into the conflict," the PLO's Zacharia al-Agha said.

Late on Sunday, several rockets were fired into the camp which has been the scene of deadly clashes between pro- and anti-regime forces over the past week, the Observatory said.

It said two men were killed by the rocket fire and another was shot dead by a sniper. (AFP)