Nearly 100 die in mass killing as rebels say they are on brink of abandoning negotiated plan to end conflict

Syria's fragile peace process is in shreds after what was claimed to be a regime-backed massacre left 32 children among more than 90 dead and triggered a wave of international revulsion. As UN observers in the central town of Houla confirmed one of the bloodiest death tolls of the 15-month revolt against President Bashar al-Assad, rebels said they were on the brink of abandoning a negotiated plan to end the conflict.

Videos uploaded to the internet and purporting to be from Houla show many dead and badly mutilated infants. Residents say some victims were killed with knives, while many more died from 18 hours of relentless shelling that left buildings wrecked and homes destroyed in a large residential area near the centre of town. Major General Robert Mood, head of the UN team in Syria, deplored the attack, which began at midday on Friday, as "indiscriminate and unforgivable" but did not say who had been to blame. Syrian state television blamed "terrorist gangs".

As calls mounted for international intervention, a peace plan negotiated in March by the former UN secretary general, Kofi Annan, appeared to be in ruins. Yesterday the main opposition Free Syrian Army, which, like regime troops had failed to abide by Annan's terms, told the TV station al-Jazeera that unless civilian safety could be guaranteed, the plan was "going to hell".

The foreign secretary, William Hague, said Britain would co-ordinate a "strong response" to the massacre. "Our urgent priority is to establish a full account of this appalling crime and to move swiftly to ensure that those responsible are identified and held to account," he said.

The international community was united in its condemnation. UN secretary general Ban Ki-moon said the killings were a "flagrant violation of international law" while the White House called the violence acts of "unspeakable and inhuman brutality." Arab League head Nabil Elaraby said the killings were a "horrific crime" and urged UN action.

Save the Children's chief executive, Justin Forsyth, echoed calls for intervention. He said: "This indiscriminate killing must stop now. The world cannot sit back and allow this to happen. Children are suffering terribly in this conflict."

Annan is due back in Damascus imminently to discuss the ongoing crisis with key officials.

Ban Ki-moon has warned there is no plan B for the situation, which poses an increasingly grave risk to regional security. Despite the chaos, UN monitors have moved widely around the country and witnessed shootings or explosions almost everywhere they have visited. In some instances, UN convoys have been targeted themselves.

The regime has touted such attacks as evidence of al-Qaida at work, while the Free Syria Army has vehemently insisted that the regime itself has been concocting the attacks in a bid to reinforce its narrative.

"Everywhere [the UN monitors] go, something happens to them," Moustafa Abdul Salam told the Observer in the northern Syrian village of Sarji.

Credit: Observer graphics

"They want to terrorise them into submission and make them doubt their own instincts. Already you have the Americans saying that al-Qaida might be at work here, so that means the regime are winning. And when they feel that way they will behave even more like savages."

Saturday's UN visit to Houla seemed to be going to script. Monitors, who arrived at about midday, were slow to engage with locals angrily remonstrating at a distance. UN officials also tried to visit Qusair south of Homs. Residents angrily claimed they had turned back for Damascus after gunfire erupted.

"This is the third time they have come here and fled," said one man contacted by Skype. "We know what happens to us if we can ever speak to one of them and they fear that the regime will attack them too."

The uprising has led to a steady unwinding of stability in the iron-clad police state coupled with a sharp rise in violence, especially since opposition groups took up arms in large numbers last August. The Free Syria Army now controls pockets of the country. Though severely under-equipped, it has the capacity to launch hit-and-run attacks against regime forces.

Opposition fighters claimed to have launched reprisal attacks in four areas of the country in the wake of the Houla attack. However, they complain that the severing of their supply lines to Lebanon will sharply limit their ability to hold the ground they now control.

Lebanon, which has been under the tutelage of Syria for much of the past 35 years, has seen an increase in sectarian tension in the past week, which is being directly linked to the crisis shaking its larger neighbour.

Last Sunday, Sunni sheikh Ahmed Abdul Wahid, a key supporter of the Syrian opposition, was killed by a Lebanese soldier at a checkpoint, leading to clashes in mixed areas of Beirut.

Events took a new twist on Wednesday when Lebanese soldiers in the heart of west Beirut were involved in a shoot-out with what officials described as a terrorist cell which it said had been trying to draw a pro-Syrian political headquarters located nearby into a firefight. Two of the men killed in the seven-hour gun battle were identified as Islamic extremists who had been freed from a Lebanese jail in November. Syria also released a large number of alleged militant Islamists from its key security prison around the same time.

"Things are never as they seem here," said a senior Lebanese member of parliament. "Al-Qaida in Beirut very much fits the storyline for Syria itself and the pro-Syrian parties in Lebanon." If the Assad regime let these people out of prison, they must have had a use for them — even an unwitting one.

"I don't doubt that these were men who thought they were on a jihad. But the question is who sent them and did they really know who their masters were?"