Activists reported heavy security presence across the northwestern province of Idlib

Arab League officials are expected to arrive in Syria later to prepare for monitors overseeing a peace plan, as activists reported more deaths in one of the deadliest assaults by security forces in the government's nine-month crackdown.

Activists in Syria on Thursday reported the deaths of at least 15 people, including 12 in the central city of Homs and three in the northwestern city of Idlib which Syria's opposition says has borne the brunt of operations by Syrian forces that left 250 people dead in the last two days.

The assault in Idlib drew international condemnation with France branding the killings an "unprecedented massacre" and the US saying Syrian authorities had "flagrantly violated their commitment to end violence".

The Turkish foreign ministry also issued a statement saying Syrian President Bashar al-Assad's government was turning the country into a "bloodbath".

It said the violence was in stark contrast to the spirit of the Arab agreement and raised doubts about Syria's "true intentions".

Nabil Elaraby, the Arab League's secretary-general, 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 the end of the month.

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.

'Bloodiest day'

Syria's opposition bloc on Wednesday reported the death of 250 people across the country in 48 hours.

Syrian National Council (SNC) demanded "an emergency UN Security Council session to discuss the [Assad] regime's massacres in Jebel az-Zawiya, Idlib and Homs, in particular" and called for "safe zones" to be set up under international protection.

On Tuesday, Syrian forces killed 111 civilians when Assad's forces surrounded them in the hills of Idlib province and unleashed two hours of bombardment and heavy gunfire, according to the Syrian Observatory for Human Rights.

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

Spotlight In-depth coverage of escalating violence across Syria

Idlib, a 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 defectors against his forces.

Al Jazeera's Zeina Khodr, reporting from Turkey, said that activists told her there was heavy security presence across Idlib and particularly in Jabal al-Jawya on Thursday.

"According to this activist, Jabal al-Zawya is under the total control of the Syrian army. Many people have fled the area. They are taking shelter in fields in that province," Khodr said.

"[Residents] were not able to escape into Turkey because the Syrian army beefed up its presence along the border."

The Syrian military has acknowledged that it was carrying out an offensive in that region.

"Last night, in a statement the military said that the operation was finished, that it had been successful. Its soldiers killed what it called armed men. It also said they managed to arrest several key men," our correspondent said.

The UN has said more than 5,000 people have been killed in Syria since the beginning of anti-Assad protests in March.