Syrian army troops have killed at least 17 people in the southern town of Deraa, among them women and children, activists say.
According to the London-based Syrian Observatory for Human Rights, Government tanks began bombarding the district before dawn.
Deraa-based activist Adel al-Omari told the Guardian: "People were taken by surprise while in their homes."
The town is where the uprising against President Bashar al-Assad began last year and comes as the UN investigates an alleged massacre in the village of al-Qubeir.
Reporters al-Qubeir have described seeing the charred remains of what could be humans or animals alongside burnt out buildings.
Sky's Tim Marshall, reporting live from the scene said he had seen bodies. "Whether it's dead animals or people, I don't know."
UN observers were shot at on Thursday as they tried to reach the scene in Hama province, days after the reported slaughter.
According to witnesses, children had their throats slit in front of their families while one report said a three-month-old baby had been burnt to death.
The BBC reports the monitors arrived on Friday after travelling in a convoy from Syrian capital Damascus.
The UN mission will attempt to verify reports coming from the country that 78 people were killed by government forces in the latest massacre by Syrian troops.
However Western journalists have suggested the delayed arrival of the monitors meant the forces behind the killings had enough time to clean up.
BBC reporter Paul Danahar tweeted he had met a man named Ahmed who claimed to witness the killings. "He has says dozens were killed," he wrote. "Butchering the people didn't satisfy the blood lust of the attackers so they killed the live stock too. Their carcasses rotting in the sun."
"The only clue to where the bodies of the people may have gone are etched into the road. UN said they were tracks made by military vehicles. Who ever did this may have acted with mindless violence but attempts to cover up the detaills of the atrocity are calculated and clear."
David Cameron has condemned the attacks, which is one of the bloodiest in the 15-month uprising against Assad
The Syrian network for Human Rights posted a video of a mother whose children were killed by the massacre on Friday. In the video, which cannot be independently verified by The Huffington Post, she says government backed forces stabbed, slaughtered and burnt children.
"They came by 3 buses and a tank. Only five or 6 people, including 3 men and 2 women, have survived the massacre. They slaughtered my children by knives. The message I want to send to the world is that the regime’s army and militias have slaughtered our men and children, and may God help us."
Reports from AP suggest government troops were shelling people in the city of Homs, one of the hardest-hit areas in the country, on Friday, as the UN monitors arrived.
Channel 4 News reporter Alex Thompson, blogging on the crisis, suggested rebels were so desperate for help they were sending journalists into areas where they may be in danger just to highlight their plight.
"In a war where they slit the throats of toddlers back to the spine, what’s the big deal in sending a van full of journalists into the killing zone?" he wrote.
The observers' arrival comes as United Nations secretary general Ban Ki-moon warned Syria is on the edge of full-scale civil war and UN envoy to the country Kofi Annan warned that "Syria is not Libya - it will not implode, it will explode, and explode beyond its borders".
Kofi Annan spoke frankly at the New York meeting, at which 193 states were present
Annan urged the UN to unite behind efforts to end the Syrian conflict on Friday and called for "consequences" if his peace plan was not implemented, blaming its failure on President Bashar Assad's government.
He said: "Clearly, the time has come to determine what more can be done to secure implementation of the plan.
"We must also chart a clearer course for a peaceful transition, if we are to help the government and opposition, as well as Syrian society, to help resolve the crisis."
The comments came as a YouGov survey for the University of Essex and Georgia State University showed less than 20% of UK and US citizens support sending troops to the country.
Essex University's Dr Thomas Scotto said attitudes could change: “Events are moving quickly and new reports of atrocities may have an impact on public attitudes. People may be swayed by coverage showing women and children being killed.
“Nonetheless, our survey suggests that opposition to using military force on the ground will remain high in both Britain and America.”
The massacre in the villages outside Hama follow some of the the worst bloodshed since the start of the UN peace plan. In May, an attack on Houla, in central Syria, resulted in the deaths of more than 90 people, including 32 children under the age of 10, according to United Nations observers.
Local activist Abu Yazan told the paper that the shelling killed 12 people, with 106 killed by pro-regime thugs called "shabiha".
Activists posted videos showing dead bodies after the massacre
Videos posted to an activist network appear to show the killings in Hama, but they have not been independently verified.
Activists have blamed pro-government militia and security forces for the deaths in the farming villages of Qubair and Maarzaf, 12 miles north of Hama. Horrific reports have emerged from activists close to the scene including one of a three-month old baby burned to death and others of men being stabbed outside their homes
Describing the attack, activists from the Syrian Network for Human Rights said that tanks rolled in "levelling half the houses". Pro-government forces then opened fire randomly on the residents of the villages, leading the men outside and "slaughtering many of them by knives".