The killing of 200 people, most of them civilians, in the Syrian village of Tremseh has been condemned as "appalling" by the international community.
UN and Arab League envoy for Syria Kofi Annan said he was "appalled" by the news of "intense fighting, significant casualties, and the confirmed use of heavy weaponry such as artillery, tanks and helicopters."
Hillary Clinton said activists' accounts that 200 people, most of them civilians, have been killed in shelling and other attacks by regime forces in Syria, were "evidence that the regime deliberately murdered civilians."
Reuters reported UN observers were prevented from entering the village to verify the reports of mass killings.
The US Secretary of State said in a statement: "We call for an immediate ceasefire in and around Hama to allow the UN observer mission to enter Tremseh.
"Those who committed these atrocities will be identified and held accountable."
WARNING, STORY CONTAINS GRAPHIC IMAGES OF DEATH
Reports suggested the village was attacked by helicopter gunships and tanks before militia carried out execution-style killings.
Syrian TV said "armed terrorist groups" took part in the massacre. Fadi Sameh, an opposition activist from Taramseh left the district before the attacks, but was in touch with residents.
He told Reuters: "It appears that Alawite militiamen from surrounding villages descended on Taramseh after its rebel defenders pulled out, and started killing the people. Whole houses have been destroyed and burned from the shelling.
"Every family in the town seems to have members killed. We have names of men, women and children from countless families."
One eyewitness statement claimed: "They blockaded the village from all four sides and began violently and randomly firing on houses as a helicopter flew overhead. As the attack happened the electricity and telephone lines were cut. Residents gathered in the streets in a state of fear and panic. They were unable to flee because of the blockade from every side."
Syrian activists have posted a video online purporting to show casualties of the massacre.
The unverified video features a line of dead bodies, including what appears to be a woman clasping a baby in her arms.
There were few other details on the attack, which was also reported by the Local Co-ordination Committees activist group and the Britain-based Syrian Observatory for Human Rights.
Reports of the incident have not be independently verified, but if they are accurate, it may represent the single biggest massacre since the uprising began.
Activists say more than 17,000 people have been killed since the uprising against President Bashar Assad began in March 2011, and he is coming under growing international pressure to stop the violence. But as the bloodshed continues, and the conflict morphs into an armed insurgency, hopes for a peaceful transition are dimming.
The latest report of violence came in the wake of the highest-level defector yet from President Bashar Assad's regime - his ambassador to Iraq.
Defections from the Syrian regime have stirred hopes in the West that Assad's inner circle will start abandoning him in greater numbers, hastening his downfall. But the tightly protected regime has largely held together over the course of the 16-month uprising, driven by a mixture of fear and loyalty.
The latest official to flee, Ambassador Nawaf Fares, announced that he was joining the revolution, asserting that only force will drive Assad from power.
Foreign Secretary William Hague said of the attacks: "Everything we have seen of the Syrian regime's behaviour over the last 17 months suggests that these reports are credible," he said.
"They demand a united response from the international community.
"We have two urgent priorities - to establish an accurate account of what happened in Traymseh so that those responsible can and will be held to account; and to agree urgent action at the United Nations Security Council.
"To that end, the UN Mission in Syria must be able to access Traymseh quickly and without hindrance so they can carry out an independent investigation into what has happened and who is responsible.
"And we will redouble our efforts to agree a Chapter Seven Resolution of the United Nations Security Council."