Envoy Kofi Annan told the Security Council the Syria crisis will "spiral out of control" if more pressure is not put on the regime, after UN monitors trying to visit the site of a new massacre were fired on.
The UN-Arab League envoy's warning came before talks on Friday with US secretary of state Hillary Clinton and as her Syria frontman meets Russian diplomats in a bid to persuade Moscow to back President Bashar al-Assad's removal from power.
Annan renewed calls for the major powers to warn Assad of "clear consequences" if he does not comply with a six-point international peace plan, one diplomat inside a closed-door council briefing at the United Nations told AFP.
"The longer we wait, the darker the future looks for Syria," another diplomat quoted Annan as telling the council.
Annan called for "united" and "substantial" pressure on Assad. He said there must be "real results soon or the crisis will spiral out of control."
He spoke after shots were fired at a UN convoy of monitors which tried to get to the village of Al-Kubeir to investigate the slaughter of civilians there.
The Syrian Observatory for Human Rights said at least 55 people were killed in Wednesday's assault on Al-Kubeir, a small Sunni farming enclave surrounded by Alawite villages in the central province of Hama.
According to preliminary evidence, the Syrian army had surrounded the village, and militia had entered Al-Kubeir and killed civilians with "barbarity," UN secretary general Ban Ki-moon was quoted as telling the Security Council.
Damascus denied responsibility for the massacre and, as it has done repeatedly in the past, pointed the finger at "terrorists" backed by foreign forces.
"A terrorist group committed a heinous crime in the Hama region which claimed nine victims. The reports by the media are contributing to spilling the blood of Syrians," state media said.
UN monitors had been prevented from entering Al-Kubeir but would make a new attempt to do so on Friday, UN spokesmen said.
After the UN meeting, Ban told reporters: "Syria can quickly go from a tipping point to a breaking point. The danger of a full-scale civil war is imminent and real, with catastrophic consequences for Syria and the region."
"The Syrian people are bleeding. They are angry. They want peace and dignity. Above all, they all want action," Ban said.
He had told the meeting that heavy weapons, armour-piercing bullets and surveillance drones have all been used against the UN Supervision Mission in Syria (UNSMIS) to hamper its efforts to monitor the worsening conflict.
The tactics had been used to try to force the unarmed monitors to withdraw from areas where government forces have been accused of staging attacks, Ban was quoted as saying.
According to UN officials, UN vehicles are shot at almost every day in Syria, and at least two roadside bombs have also targeted UN convoys.
Ban praised the "brave" monitors but said the Security Council would have to consider whether the mission is "sustainable".
UN observers had seen Syrian military convoys approaching villages and tried to stop tank assaults against populated areas, but had been "ignored," Ban said.
The Security Council has passed two resolutions which approved the UN monitoring mission in Syria and condemned the violence there, but it is divided over how to increase pressure on the regime.
Russia, Syria's last major ally, and China have vetoed two council resolutions which only hinted at future sanctions. The United States and European nations want economic sanctions against Assad.
Annan said he was in discussions to set up an international contact group on the Syria crisis and that he hoped Iran would be part of the "solution."
But US ambassador to the UN Susan Rice said Iran was a "spoiler" and "part of the problem in Syria."
"There is no question that it is actively engaged in supporting the government in perpetrating the violence on the ground," she told reporters.
"Iran has not demonstrated to date a readiness to contribute constructively to a peaceful political solution."
On the political front, Annan was to meet Clinton in Washington on Friday to discuss Syria, as her point man on the conflict Fred Hof visited Russia.
Russia and China have vetoed two Security Council resolutions against Assad's regime, but backed Annan's blueprint to end a conflict in which the Observatory says more than 13,500 people have died since March 2011.
Russian foreign minister Sergei Lavrov vowed there would be no UN Security Council mandate for outside intervention in Syria, indicating Moscow would use its veto to block any military action.