Arab League officials arrive in Syria on Thursday to prepare for monitors overseeing an Arab peace plan, after activists said President Bashar al-Assad's forces carried out the deadliest assault in their nine-month crackdown on protests.

The Syrian Observatory for Human Rights said Syrian forces killed 111 civilians and activists were killed on Tuesday when Assad's forces surrounded them in the hills of Idlib province and unleashed two hours of bombardment and heavy gunfire.

France branded the killings an 'unprecedented massacre' and the United States said Syrian authorities had 'flagrantly violated their commitment to end violence'.

Another 100 army deserters were either wounded or killed, making it the "bloodiest day of the Syrian revolution", the British-based Observatory's director Rami Abdulrahman said.

Events in Syria are hard to verify because authorities, who say they are battling terrorists who have killed more than 1,100 soldiers and police, have banned most independent reporting.

Tuesday's bloodshed brought the death toll reported by activists in the last 48 hours to over 200.

The main opposition Syrian National Council said 'gruesome murders' were carried out, including the beheading of a local imam, and demanded international action to protect civilians.

The escalating death toll in nine months of popular unrest has raised the spectre of civil war in Syria with Assad, 46, still trying to stamp out protests with troops and tanks despite international sanctions.

Idlib, a northwestern province bordering Turkey, has been a hotbed of protest during the revolt, inspired by uprisings across the Arab world this year, and has also seen increasing attacks by armed insurgents against his forces.

A politician in neighbouring Lebanon said Assad was trying to crush opposition in the area before the arrival of the monitors, to prevent any de facto 'buffer zone' emerging near the Turkish border.

The Observatory said rebels had damaged or destroyed 17 military vehicles in Idlib since Sunday while in the southern province of Deraa violence continued on Wednesday.

Tanks entered the town of Dael, the British-based group said, leading to clashes in which 15 security force members were killed. Six army defectors and a civilian also died and dozens of civilians were wounded, it said.

ARAB PEACE MONITORS
The Syrian National Council said 250 people had been killed on Monday and Tuesday in 'bloody massacres', and that the Arab League and United Nations must protect civilians.

It demanded "an emergency U.N. Security Council session to discuss the (Assad) regime's massacres in Jabal al-Zawiyah, Idlib and Homs, in particular" and called for 'safe zones' to be set up under international protection.

It also said those regions should be declared disaster areas and urged the International Red Crescent and other relief organisations to provide humanitarian aid.

Arab League Secretary-General Nabil Elaraby said on Tuesday that an advance observer team would go to Syria on Thursday to prepare the way for 150 monitors due to arrive by end-December.

Syria stalled for weeks before signing a protocol on Monday to admit the monitors, who will check its compliance with the plan mandating an end to violence, withdrawal of troops from the streets, release of prisoners and dialogue with the opposition.

Syrian officials say over 1,000 prisoners have been freed since the plan was agreed six weeks ago and that the army has pulled out of cities. The government promised a parliamentary election early next year as well as constitutional reform which might loosen the ruling Baath Party's grip on power.

Syrian pro-democracy activists are deeply sceptical about Assad's commitment to the plan, which, if implemented, could embolden demonstrators demanding an end to his 11-year rule, which followed three decades of domination by his father.

Assad is from Syria's minority Alawite sect, an offshoot of Shi'ite Islam, and Alawites hold many senior posts in the army which he has deployed to crush the mainly Sunni Muslim protests.

In recent months, peaceful protests have increasingly given way to armed confrontations, often led by army deserters.

The United Nations has said more than 5,000 people have been killed in Syria since anti-Assad protests broke out in March.