The world community piled the pressure on Syria Thursday to end 17 months of bloodshed as rights groups accused Damascus of a new atrocity and the conflict threatened to entangle neighbouring Lebanon.

The Organisation for Islamic Cooperation (OIC) suspended Syria, saying it can no longer accept a regime that "massacres its people", and even traditional ally China told Damascus to rapidly implement a ceasefire.

The latest moves come after dozens of people, including women and children, were reported killed in a massive air strike on civilians in a rebel bastion in the north of Syria.

A UN panel said Wednesday that Syrian forces and their militia allies had committed crimes against humanity, including the shocking Houla massacre, during a conflict that has killed thousands and sent many more fleeing.

Violence continues to grip many parts of the country, including the northern battleground of Aleppo, near the scene of Wednesday's air strike in the town of Aazaz.

The UN Security Council meets later Thursday to formally end its observer mission in Syria with world powers still in deadlock over how to end a conflict that risks destabilising the entire region.

Ahead of the meeting, Chinese Foreign Minister Yang Jiechi -- whose country has joined Russia in vetoing three UN resolutions on the crisis -- urged Damascus to implement a ceasefire and accept international mediation.

"China urges the Syrian government and all concerned parties... to quickly implement a ceasefire to end the violence and start political dialogue," Yang told visiting Syrian envoy Bouthaina Shaaban.

Adding to the pressure on the embattled regime, an emergency OIC summit said it had agreed to suspend Syria because of "deep concern at the massacres and inhuman acts suffered by the Syrian people".

OIC chief Ekmeleddin Ihsanoglu said: "This (Muslim) world can no longer accept a regime that massacres its people using planes, tanks and heavy artillery."

The United States and the opposition Syrian National Council welcomed the move, but it was rejected by the official press in Damascus and Syria's staunch ally Iran.

"Today's action underscores the Assad regime's increasing international isolation and the widespread support for the Syrian people and their struggle for a democratic state that represents their aspirations and respects their human rights," State Department spokeswoman Victoria Nuland said.

The West is pushing hard for the departure of Assad, whose inner circle has been hit by a wave of defections and a rebel bomb attack that took out four of his top security officials last month, but is at odds with Russian and China.

A damning report by the UN Commission of Inquiry issued Wednesday said government forces and their militia allies committed crimes against humanity including murder and torture, while the rebels had also carried out war crimes, but on a lesser scale.

"The commission found reasonable grounds to believe that government forces and the shabiha had committed the crimes against humanity of murder and of torture, war crimes and gross violations of international human rights law and international humanitarian law," the report said.

It said they were responsible for the massacre in the central city of Houla in May when 108 civilians, including 49 children, were killed in a grisly attack that Assad himself had said was the work of "monsters".

Rebel fighters were however not spared in the probe, which found them guilty of war crimes, including murder, extrajudicial execution and torture.

In the north of Syria, activists and residents reported another atrocity by the regime, with dozens killed Wednesday in an air strike in Aazaz, a rebel bastion near Aleppo.

The Syrian Observatory for Human Rights said 31 people were killed, including women and children, and another 200 wounded, while Turkey, which took in many of the victims, said 15 had died of their injuries.

"Bashar did this. God help us, these animals will kill us all," said one man, hoisting a bloodied arm from a pile of body parts on the pavement outside the local hospital.

Dozens of residents fled for nearby Turkey, many of them entire families carrying boxes of clothing and food on their heads.

Human Rights Watch urged the Security Council to impose an arms embargo on Syria after the air strike on Aazaz.

"Yet again, Syrian government forces attacked with callous disregard for civilian life," said Anna Neistat, HRW acting emergencies director.

UN humanitarian chief Valerie Amos also warned that the situation in Syria was worsening, with the number of people in need possibly as high as 2.5 million and at least a million at risk of "destitution".

At least 80 people were killed nationwide Thursday, including 18 civilians in shelling in Aleppo, according to the Observatory.

The conflict erupted in March last year when regime forces cracked down on peaceful protests but has spiralled into an armed rebellion that the Observatory says has killed 23,000 while the UN puts the toll at 17,000.

Stoking fears that the conflict could spread, dozens of Syrians were kidnapped in Lebanon on Wednesday -- many by a Shiite Muslim clan -- in retaliation for events across the border.

"This brings us back to the days of the painful war, a page that Lebanese citizens have been trying to turn," Prime Minister Najib Mikati said, recalling the dark days of the civil war and the kidnapping of Western hostages.

Saudi Arabia -- the regional Sunni powerhouse that is opposed to Assad's Alawite-led regime -- along with at least two other Gulf states ordered their nationals to leave immediately because of threats.

Lebanon has in the past had to confront cross-border shootings, shelling by the Syrian army, tit-for-tat kidnappings and sectarian clashes as the violence in Syria escalates.

In New York, the Security Council meets to formally end its observer mission with UN chief Ban Ki-moon -- who has branded the conflict a "proxy war" -- calling for a "flexible" UN presence in future.