THE Syrian army stepped up attacks on rebels yesterday, bombarding the city of Douma, near Damascus.

More violence erupted yesterday in a 16-month conflict that opposition leaders say has killed more than 15,000 people.

"There was heavy shelling all morning, now it has calmed down a bit but the siege of the city continues," said Abu Rami, an opposition activist in Homs, which has borne the brunt of an army onslaught on rebel strongholds.

The army shelled towns and villages near Douma, to which the embattled city's residents had fled at the weekend, according to Omar Hamzeh, spokesman for the revolutionary council of rural Damascus. He said at least six people had been killed.

Meanwhile, Turkey said it had scrambled F-16 fighters the previous day after Syrian helicopters flew near its border.

Turkey's armed forces command said the jets took off on Monday when Syrian transport helicopters were spotted flying near the frontier, without entering Turkish air space. It was the third day in a row Turkey had scrambled its F-16s.

Syrian President Bashar al Assad told a Turkish newspaper he wished his forces had not shot down a Turkish jet last month and that he would not allow tensions with Turkey to lead to war.

The downing of the Turkish F4 in disputed circumstances has aggravated hostility between Damascus and Ankara, which has raised its military profile to try to keep Syrian helicopters from Turkey's border zone where rebels and refugees are camped.

"We learned it belonged to Turkey after shooting it down. I say 100% 'if only we had not shot it down'," Turkey's Cumhuriyet daily quoted Mr Assad as saying.

A Syrian general and 84 soldiers were the latest to defect and flee to Turkey on Monday. But army and government defections have so far failed to shake Mr Assad's 12-year grip on power.

Diplomacy has so far failed to stem the bloodshed. World powers agreed at the weekend to back talks on a transitional government. But they failed to narrow differences between the West and Russia over Western demands that Mr Assad must go.

Amid the devastation, Human Rights Watch has claimed in a report that Syrian intelligence agencies are running a network of torture centres across the country where detainees are beaten with batons and cables, burned with acid and sexually assaulted.

The state-sanctioned abuses amounted to crimes against humanity that should be investigated by the International Criminal Court, the New York-based campaign group added. Its report identified 27 detention centres it said intelligence agencies have been using since March 2011 when President Assad's government began a crackdown on pro-democracy protests that have grown into an armed revolt.

Syria's government did not respond to the accusations which have been echoed in earlier reports by the United Nations.

Human Rights Watch said tens of thousands of people had been detained by Syria's Department of Military Intelligence, the Political Security Directorate, the General Intelligence Directorate and the Air Force Intelligence Directorate. The group said it conducted more than 200 interviews with people who said they were tortured.

The group said it documented more than 20 torture methods that "point to a state policy of torture and ill-treatment and constitute a crime against humanity".