AMMAN/BEIRUT (Reuters) - The United States has branded Syria's leaders murderers after an attack on a village by President Bashar al-Assad's troops left dozens dead, but there was no break in the deadlock among world powers over how to bring about an end to the bloodshed.
United Nations Secretary-General Ban Ki-moon also condemned what his monitors on the ground had seen as an "indiscriminate" bombardment that included rocket-firing helicopters of the town of Tremseh in rebellious Hama province, and he questioned Assad's commitment to a U.N.-sponsored peace plan for Syria.
But at U.N. headquarters in New York, U.S. diplomats and their Western allies continued to run up against a refusal on the part of Russia to lift its veto on harsher sanctions or any steps Moscow views as imposing "regime change" in Damascus.
There was "indisputable evidence that the regime deliberately murdered innocent civilians", said U.S. Secretary of State Hillary Clinton, demanding access for U.N. observers who on Thursday were spectators to hours of bombing and gunfire, but were kept out of the village by Syrian troops.
As video evidence of casualties from the attack on the village emerged on the Internet, Ban said: "I condemn, in the strongest possible terms, the indiscriminate use of heavy artillery and shelling of populated areas, including by firing from helicopters."
Accounts from opposition activists cited a death toll ranging from over 100 to more than twice that figure - either way one of the bloodiest incidents in 17 months of conflict.
One group said rebel fighters rushed to reinforce the village after it came under attack by infantry, artillery and air forces, leading to a battle that went on for seven hours.
In a pattern seen at other settlements in recent months, rebels accused irregular militiamen, known as shabbiha, from Assad's Alawite minority, of swooping on the battered village, home mostly to Sunni Muslims, and of finishing off their neighbours in a sectarian attack some called ethnic cleansing.
Syrian state television accused "armed terrorist groups" of committing a massacre at Tremseh, but gave no death toll.
Assad, who succeeded his late father 12 years ago, has plenty of firepower to suppress the opposition and can count on backing both from Shi'ite Iran, hostile to the Sunni Arabs who lead most states in the region, and has also been protected from sanctions by Syria's old Cold War ally Russia.
Moscow rejects Western governments' insistence that Assad must go and says a peace process must come from within Syria. It is hosting the U.N. special envoy, Kofi Annan, at the Kremlin next week, as diplomats at the Security Council will be resuming efforts to narrow differences over raising pressure on Damascus.
Annan called the events at Tremseh a "grim reminder" that U.N. resolutions calling for peace were being flouted and wrote to the Council urging it to penalise Syria for failing to comply. But talks on Friday showed little progress.
One senior Western diplomat said: "The problem is Russia.
"I'm not saying they are not working behind the scenes, but clearly it hasn't worked and they have to admit that either they haven't been pushing Assad hard enough or they have and they have failed to persuade him ... At the moment, the effect of what they are doing, maybe not the intention, but the effect, is just to give space for the massacres to continue."
French President Francois Hollande said he was urging Russia - and China, which shares Moscow's suspicion of Western powers' appetite for intervening in sovereign affairs - to change tack.
"A regime has decided to use force to crush its own people," he said. By blocking sanctions that might force Assad aside, Russia and China would let "chaos and war take hold in Syria" in way that would harm their own interests.
Critics of the approach by Western powers, loath to undertake direct military involvement of their own, say the revolt in Syria could lead to a wider, sectarian conflict and bring to power radical Islamists hostile to non-Arab nations.
Washington and its European and Arab allies are wary of the rebel forces, which have proved fractious among themselves, but believe an erosion of support for Assad within the elite - as seen in high-level defections in the past week - may in time allow for a period of political transition without him.
Clinton appealed to the veto-wielding Russians and Chinese on the Security Council to join the Western drive against Assad. "History will judge this Council," she said in a statement.
"Its members must ask themselves whether continuing to allow the Assad regime to commit unspeakable violence against its own people is the legacy they want to leave."
Russia withheld blame while condemning the violence and called for an inquiry into events at Tremseh. "This wrongdoing serves the interests of those powers that are not seeking peace but persistently seek to sow the seeds of interconfessional and civilian conflict on Syrian soil," the foreign ministry said.
A local activist named Ahmed told Reuters there were 60 bodies at the mosque, of whom 20 were identified: "There are more bodies in the fields, bodies in the rivers and in houses."
Opposition video segments posted on YouTube provided evidence that dozens had met a violent death.
One piece of film to appear on the Internet showed the corpses of 15 young men with faces or shirts drenched in blood. Most wore T-shirts and jeans. There were no women or children.
Other videos showed rows of bodies wrapped in blankets, sheets and makeshift shrouds, some leaking blood. One man pulled aside a blanket to display a carbonised corpse. Men placed wrapped bodies in a breeze-block trench for burial.
In a mosque packed with grieving women and distraught men, bodies were collected, identified and prepared. Children stepped gingerly among the corpses covering the floor.
Additional by Stephanie Nebehay in Geneva, Michelle Nichols at the United Nations and John Irish in Paris; Writing by Alastair Macdonald; Editing by Mark Heinrich