(Adds U.N. observers in Tremseh and Syria, Iran response)
* U.N. monitors at site of Tremseh attack
* 33 dead in fresh violence
* China condemns mass killing but attributes no blame
* U.N.'s Ban questions Assad commitment to peace plan
* Russia rejects moves to oust Assad
By Oliver Holmes
BEIRUT, July 14 (Reuters) - U.N. observers entered the central Syrian village of Tremseh on Saturday, two days after activists said about 220 people had been killed there by helicopter gunships and militiamen, prompting international outrage.
The United States has branded Syria's leaders murderers after the attack by Syrian President Bashar al-Assad's troops, but there was no break in the deadlock among world powers over how to bring about an end to the bloodshed.
U.N. Secretary-General Ban Ki-moon condemned what a U.N. reconnaissance mission on Friday said was "indiscriminate" bombardment of the central Hama province village, including rocket-firing helicopters. He questioned Assad's commitment to a U.N.-sponsored peace plan for Syria.
The U.N. spokeswoman in Damascus, Sausan Ghosheh, confirmed a team of observers sent to Tremseh on Saturday to gather facts about the killings had arrived in the village.
Syria rejected allegations of a massacre and said the attack on Thursday was a successful military operation that killed a large number of "terrorists" but no civilians.
The bloodshed in the country continued on Saturday, when British-based Syrian Observatory for Human Rights said 33 people were killed, several by an army bombardment in Homs province.
Accounts from opposition activists cited a death toll in Tremseh ranging from over 100 to more than twice that - one of the bloodiest incidents in the 17-month uprising against Assad that Western powers say has left 17,000 dead.
"We were surrounded from four sides ... with tanks and armoured vehicles, and the helicopters were hovering above," said an unidentified man on video footage purportedly filmed in Tremseh and posted on the Internet on Saturday.
"They burned people in front of our eyes, they held the men like this and stabbed them," he said, pointing to his chest and then to an artery in his throat. He said his cousin's throat was slit. "They took out people's eyes."
One group said rebel fighters rushed to reinforce the village after it came under attack by infantry, artillery and aircraft, leading to a battle that lasted seven hours.
In a pattern seen elsewhere in recent months, rebels accused local irregular militiamen known as shabbiha, from Assad's Alawite minority, of swooping on the battered village, home mostly to Sunni Muslims, and of killing their neighbours in a sectarian attack some called ethnic cleansing.
A Tremseh 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."
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 burnt 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.
Quoting an unnamed military source, Syrian state news agency SANA said the Tremseh operation came in response to the "pleas of the inhabitants of Tremseh who had been exposed to various forms of criminal acts at the hands of armed terrorist groups".
It said the residents of Tremseh thanked the authorities for "restoring security and safety to the area".
While Washington laid the blame for the killings at Assad's door, China said it strongly condemned "behaviour which harms innocent civilians" but did not say who it believed carried out the attack.
"We again urge all relevant sides in Syria to take practical steps to immediately stop all violence," Chinese Foreign Ministry spokesman Liu Weimin said in a short statement.
China and Russia continue to block Western efforts to impose harsher sanctions on Syria or take any steps Moscow and Beijing view as supporting "regime change" in Damascus.
Moscow rejects the Western insistence that Assad must go and says a peace process must come from within Syria. It is hosting U.N. special envoy Kofi Annan next week, as diplomats at the Security Council will resume efforts to narrow differences over raising pressure on Damascus.
Annan called 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.
French President Francois Hollande said on Saturday there was still time to find a political solution to avoid a civil war in Syria, and urged Russia to stop blocking efforts for a resolution at the Security Council.
"I told (Russian President) Vladimir Putin that the worst thing that could happen is a civil war in Syria so let's work together to find a political solution to avoid civil war. There is still time," he said in a television interview.
Critics of Western powers' reluctance to undertake direct military action say they are using Russia as a scapegoat.
Iran, Assad's main backer, said on Saturday it would take a more hands-on approach to international efforts to resolve the conflict.
"We are ready to have a proper role in establishing stability and security in all countries in the region, including Syria," Iranian Foreign Ministry spokesman Ramin Mehmanparast said in comments published in "Iran" newspaper.
Washington and its European and Arab allies are wary of the rebel forces, which have proved fractious, 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.
(Additional reporting by Erika Solomon in Beirut, Khaled Yacoub Oweis in Amman, Stephanie Nebehay in Geneva, Michelle Nichols at the United Nations, John Irish in Paris and Yeganeh Torbati in Dubai; Editing by Pravin Char)