THE Syrian conflict spilled over into Turkey yesterday as forces loyal to President Bashar al Assad battled rebel fighters near the border in clashes that could doom a planned ceasefire.

Under a deal brokered by United Nations-Arab League envoy Kofi Annan, Syria is to begin withdrawing troops from urban centres today, paving the way for a truce on Thursday.

However, the prospect of a ceasefire looked increasingly dim, with no let-up in violence around the country where Government troops have battled to crush a popular revolt against Mr Assad's rule for more than a year.

In a last-minute move, Mr Assad demanded written guarantees from opposition fighters that they put down their weapons, prompting Turkey's Deputy Foreign Minister Naci Koru to say today's deadline for withdrawal was now effectively void.

Fierce fighting raged on the ground, with two Syrian refugees and a Turkish translator wounded by Syrian gunfire at a refugee camp in Turkey.

In clashes inside Syria near the Turkish border, rebels killed six members of Mr Assad's security forces, according to the Syrian Observatory for Human Rights. It said the fighting took place in the village of Salama, between the Syrian town of Azaz and the Turkish town of Kilis. Eight rebel gunmen were wounded.

In Damascus province, four soldiers died in a bomb attack as Syrian forces swept villages arresting opposition suspects. Two policemen were killed in clashes with gunmen in Aleppo, the UK-based Observatory said.

Colonel Qassem Saad al Deen, spokesman for the Free Syrian Army, said at least 1000 people had been killed during the past week, most of them civilians.

Western powers suspect Assad has used the time since he met Mr Annan in Damascus a month ago to mount a military offensive aimed at clearing the country of rebel strongholds.

A human rights group claimed yesterday that Syrian security forces have executed more than 100 civilians and rebel fighters.

A Human Rights Watch report documents more than a dozen incidents involving at least 101 victims since late 2011, many of them in March 2012, in which Syrians were killed in cold blood by security forces and pro-Government militiamen. Ole Solvang, emergencies researcher at Human Rights Watch, said: "They are doing it in broad daylight and in front of witnesses, evidently not concerned about any accountability for their crimes. Syrian security forces will stop the executions only if they sense that accountability is inevitable. It is up to the Security Council to send this message."

Thirteen bodies were found dumped on the street in Homs on Saturday. Locals said it was a massacre by Assad supporters.

The victims, all young or middle-aged men, had their hands tied behind their backs. Some were blindfolded.

The United Nations says 9000 people have been killed since the uprising started in March 2011.

The severity of Mr Assad's crackdown has triggered Western condemnation and sanctions. The Syrian economy has ground to a halt and its pound has halved in value.