UN monitors are to try again on Friday to reach the site of a new Syrian massacre after being shot at in their first attempt, after Kofi Annan urged united action to stop the crisis from spiralling out of control.

Anti-regime demonstrations were expected after weekly Muslim Friday prayers, activists said, including in the capital.

In a closed-door briefing to the UN Security Council, the UN-Arab League envoy called on world powers to warn President Bashar al-Assad's regime of "clear consequences" unless it abides by his widely flouted peace plan, a diplomat told AFP.

"The longer we wait, the darker the future looks for Syria," Annan was quoted as saying.

Annan called for "united" and "substantial" pressure on Assad, and said there must be "real results soon or the crisis will spiral out of control."

His remarks came after at least 55 people were killed on Wednesday in an assault on Al-Kubeir, a Sunni farming enclave surrounded by Alawite villages in the central province of Hama, the Syrian Observatory for Human Rights said.

A convoy of UN observers was fired on when it tried to investigate the slaughter on Thursday and was set to make a new attempt to enter the village on Friday, the UN said.

According to preliminary evidence, Syrian soldiers had surrounded Al-Kubeir and militia had entered the village and killed civilians with "barbarity," UN chief 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, blamed foreign-backed "terrorists."

"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.

The new UN attempt to visit the site comes as Annan holds 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 Assad's removal.

It comes despite the almost daily targeting of the UN Supervision Mission in Syria (UNSMIS), including the use of heavy weapons, armour-piercing bullets and surveillance drones to hamper its efforts, according to Ban.

The observers had seen military convoys approaching villages and tried to stop tank assaults against populated areas, but had been "ignored," said the UN chief.

At least two roadside bombs have targeted their convoys since the observers first began deploying in mid-April.

Such tactics had been used to try to force the unarmed monitors to withdraw from areas where government forces have been accused of staging attacks, the UN chief was quoted as saying.

Ban praised the "brave" monitors, but said the Security Council would have to consider whether the mission is "sustainable."

"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," he told reporters.

"The Syrian people are bleeding. They are angry. They want peace and dignity. Above all, they all want action."

On Friday, troops battled to take back the rebel bastion of Khaldiyeh in the central city of Homs, bombarding the district with shells, said the Observatory.

Khaldiyeh was pounded "at a rate of five shells a minute" at one stage, the Britain-based watchdog said.

Elsewhere, a blast killed at least two security forces members in the northwestern province of Idlib, where a civilian was also shot dead, it added.

At least 58 people were killed across Syria on Thursday.

New protests have been called Friday under the rallying cry "Revolutionaries and traders, hand in hand until victory.".

Large protests in Damascus were set to begin after Friday prayers, an activist in the city who uses the pseudonym Deeb Dimashqi told AFP via Skype.

"This morning, small protests took place just after dawn prayers in several areas of the city. That was in spite of heavy security deployment," he said.

More than 13,500 people have been killed in the crackdown on dissent that followed the eruption in mid-March 2011 of anti-government protests and the increasingly violent insurgency against Assad's regime, the Observatory says.

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.

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.

At a recent Security Council meeting Rice likened the monitors to "300 sitting ducks in a shooting gallery, one IED from a disaster."

Russia, Syria's last major ally, and China have vetoed two Security Council resolutions against Assad's regime but backed Annan's blueprint to end the conflict.

Russian Foreign Minister Sergei Lavrov has vowed there would be no Security Council mandate for outside intervention in Syria, indicating Moscow would again use its veto to block any military action.

China on Friday condemned the latest civilian killings in Syria, but refused to back Annan's call to increase the pressure on the Assad regime.