BEIRUT, Lebanon - 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 al-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.

Members of Syria's opposition said the bloodshed outside the village of Kfar Owaid, about 30 miles 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."

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 antigovernment protests.

One Kfar Owaid villager who is an antigovernment activist said 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 antiregime protests recently in the village.

Those who fled to the valley were 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.

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.