What activists are calling the worst of three large-scale massacres in Syria since late May is sparking widespread outrage, with U.N. envoy Kofi Annan singling out the Syrian government for the violence and the opposition demanding swift international action.

With unusual candor, Mr. Annan Friday blamed government forces and armed militiamen backed by tanks and helicopters for slaughtering about 220 people in the Sunni farming village of Tremseh, in rebellious Hama province.

Condemning what he called “atrocities” and voicing shock at the “intense fighting, significant casualties and the confirmed use of heavy weaponry,” Mr. Annan said the Syrian government had violated its commitments to the peace plan he brokered in April.

Rebel backers claim the attack on Thursday was the worst single act of violence since the country's revolt against President Bashar al-Assad began in March 2011.

The Syrian Muslim Brotherhood also blamed Mr. Assad for the reported massacre and what it called Syria's foreign backers, including Russia, Iran and Mr. Annan. Protests broke out in multiple cities across Syria Friday, with demonstrators condemning the killings in Tremseh.

Syrian state media blamed the massacre on dozens of terrorists who overran Tremseh, killing and wounding dozens of civilians. It said the attackers “ransacked, destroyed and burned scores of houses” before “competent authorities” arrived.

Norwegian Gen. Robert Mood told journalists in Damascus that U.N. observers based in Hama witnessed “continuous fighting [Thursday] in the area of Tremseh.”

Mood added that observers were prepared to enter Tremseh once a cease-fire is in place:

“If we have credible cessation of violence and a local cease-fire, we stand ready to go in with a larger team to verify the facts on the ground. What we have verified today is that there was on-going fighting involving mechanized units, involving indirect fire impact and involving helicopters. Will it be more possible to verify more facts in the coming hours and days? I don't know. I sincerely hope so.”

The White House said further “atrocities” in Syria by Mr. Assad's government should end any doubts on the need for a coordinated international response at the United Nations.

The Syrian National Council, the country's main exiled opposition group, called on the Security Council to “assume total responsibility to protect defenseless Syrians against these shameful crimes.”

At a news conference in Istanbul, opposition leader Abdel Basset Syeda said responsibility for the massacre goes to the Syrian government's ally Russia, which he says has become “part of the conflict.” Friendly nations, Syeda said, must “move from words to action” and “show real friendship.”

Syeda said the Syrian opposition is sending a delegation to the U.N. to press for quick action.

Middle East analyst Nadim Shehadi of Chatham House in London told VOA it is a mistake for the international community to wait.

“Non-intervention bears a higher cost than intervention. It is wrong to wait for a massacre that is big enough to trigger an intervention. The international community, by not protecting the civilians, are turning what was originally a non-violent revolt against the regime into a civil war.”

Rafif Jouejati, a spokeswoman for the Local Coordination Committees, an activist group with members throughout Syria, told VOA the death toll from Thursday's attack is likely to rise as more casualties are confirmed. She contends a “consistent pattern” has emerged, where each attempt by the international community to reach a political solution in Syria is met by the Damascus government with a calculated escalation in violence.

Another activist group, the Britain-based Syrian Observatory for Human Rights, said it has confirmed the names of 63 people killed in the attack on Thursday, and expects the final toll to be far higher.

VOA cannot confirm events on the ground in Syria because the government severely restricts access for international journalists.

The Revolution Leadership Council of Hama said Tremseh was first shelled, then invaded by pro-government militiamen of Mr. Assad's Alawite sect, who swept in and killed victims one by one. It said some civilians were killed while trying to flee.

Multiple reports quoting activists and witnesses said a convoy of vehicles from nearby Alawite villages surrounded Tremseh early Thursday. Militiamen then blockaded the settlement and began randomly firing on houses as a helicopter flew overhead. Electricity and telephone lines were cut. Bodies were later recovered from fields, private homes and the nearby Orontes River.

The previous largest massacre in Syria took place in nearby Houla, when 108 men, women and children were killed on May 25 – the vast majority of them executed, according to a United Nations report. In June, it took U.N. monitors two days to reach the site of an alleged massacre of 78 people shot, stabbed or burned alive in the Sunni hamlet of Mazraat al-Qubeir by pro-government shabiha militiamen.

Most Assad loyalists – including the shabiha – are minority Alawites, who form a branch of Shi'ite Islam. The street protesters and fighters behind the uprising are mostly Sunni Muslims.

Activist groups have put the death toll in the 16-month-old conflict at more than 17,000 people.

Meanwhile, the United States says the international community will hold Syrian officials accountable if they fail to meet their duty to safeguard the country's stockpiles of chemical weapons.

State Department spokeswoman Victoria Nuland said the U.S. has repeatedly warned Syria it is obligated to protect those weapons. She made her comments after The Wall Street Journal reported that Syria has started moving part of its chemical weapons arsenal out of storage facilities. The Journal quoted U.S. officials as saying Syria may use the weapons against rebels or civilians.

Syria is believed to have reserves of sarin nerve agent, mustard gas and cyanide.

Throughout the conflict in Syria, the newspaper said, U.S. officials and their allies in the region have been watching for changes in the status or location of Syria's alleged chemical weapons.