As government troops advanced on a village in northwestern Syria, activists say the terrified residents fled into a valley for fear of being arrested or worse. What happened next, one of the activists said, was "an organized massacre."

The troops surrounded the valley and unleashed a barrage of rockets, tank shells, bombs and gunfire in an hours-long assault, according to two human rights groups and a witness, killing more than 100 people and leaving no survivors in one of the bloodiest days of a crackdown by President Bashar Assad against a nine-month popular uprising.

The White House said it was "deeply disturbed" by Tuesday's attack, France called it a "murderous spiral," and the Arab League reminded the Assad regime of its responsibilities to protect its civilians.

The United Nations says more than 5,000 people have died since March as Syria has sought to put down the uprising — part of the Arab Spring of protests that has toppled long-serving unpopular leaders in Tunisia, Egypt and Libya.

Anti-government protesters attend the funeral of protesters killed in earlier clashes in the Damascus suburb of Zabadani on Wednesday. (Reuters)

Members of Syria's opposition said the bloodshed outside the village of Kfar Owaid, about 50 kilometres from the northern border with Turkey in Idlib province, was evidence of the authoritarian leader's intent to intensify its crackdown on the uprising before Arab League observers arrive in the country Thursday. The death toll from two days of violence this week topped 200, including up to 70 army defectors killed near the city of Idlib, the activists said.

"It was an organized massacre," said Rami Abdul-Rahman, head of the Syrian Observatory for Human Rights, a British-based activist group. "The troops surrounded people, then killed them."

Residents fled

Kfar Owaid is part of the rugged mountainous region of Jabal al-Zawiyah, the scene of clashes between troops and army defectors, as well as weeks of intense anti-government protests.

One Kfar Owaid villager who is an anti-government activist told The Associated Press by telephone that scores of residents and activists had fled to the nearby Budnaya Valley ahead of the advancing troops. He said the security forces had lists of names of those who organized massive anti-regime protests recently in the village.

Those who fled to the valley were completely surrounded by troops, said the activist, who identified himself only as Abu Rabih for fear of government reprisal. The troops then opened fire with tanks, rockets and heavy machine-guns, he said, adding that they also used bombs filled with nails to increase the number of casualties.

He said 110 people were killed in the attack, with 56 of them buried in Kfar Owaid on Wednesday. Others were buried in nearby villages.

Abdul-Rahman corroborated the Kfar Owaid witness account. The group, which uses a network of local activists to collect information on the crackdown, said 111 people from the village were killed Tuesday.

All of those in the valley were unarmed civilians and activists, with no armed military defectors among them, the rights groups said.

The Jabal al-Zawiyah region has been under intense attack by government forces since Saturday, Abu Rabih said.

Syrian officials have not commented on the allegations.

Arab League plan

Assad agreed Monday to allow foreign monitors into Syria under an Arab League plan aimed at stopping the bloodshed. The huge toll Monday and Tuesday from the crackdown has reinforced opposition suspicions that Assad is trying to stall before a new round of international condemnation and sanctions. His regime already has been left isolated and under pressure from the Arab world as well as the West.

Syrian soldiers chant slogans during a rally at Umayyad Square in Damascus on Wednesday. (Muzaffar Salman/Associated Press)

In Cairo, Arab League chief Nabil Elaraby expressed deep concern about reports of an escalation in violence and appealed to Damascus to shoulder its responsibilities to protect civilians in compliance with its pledges to abide by the league's plan.

The Arab League plan calls for Syria to halt its crackdown, open talks with the opposition, withdraw military forces from city streets and allow in human rights workers and journalists. The 22-member Arab League has also suspended Syria's membership and levelled economic and diplomatic sanctions.

Despite the new violence, the Arab League appeared to be going ahead with its plans to send in its first delegation of monitors on Thursday. An Arab League official said the second team of observers — 30 experts in military affairs and human rights — will head for Syria on Sunday, led by Lt. Gen. Mohammed Ahmed Mustafa of Sudan.

Another team of 100 observers will leave for Syria within two weeks, he said.

The Obama administration, meanwhile, ramped up its criticism of the Syrian government Wednesday, accusing it of continuing to "mow down" its citizens despite promises to halt a brutal crackdown on reformers. The White House said Assad cannot be trusted, does not deserve to rule and must leave power.

The new barrage of criticism came as the U.S. State Department issued a new travel warning for Syria repeating earlier calls for Americans to leave the country immediately and advising that it will further reduce its staff at the U.S. Embassy in Damascus.

"The words of the Assad regime have no credibility when they continue to be followed by outrageous and deplorable actions," White House spokesman Jay Carney said in a statement. "They have already flagrantly violated their commitment to end violence and withdraw security forces from residential areas," he said.

"The United States continues to believe that the only way to bring about the change that the Syrian people deserve is for Bashar al-Assad to leave power," Carney said.