DAMASCUS (AFP) -- International chemical weapons experts were preparing to launch an ambitious and dangerous disarmament mission in Syria on Sunday even as its relentless conflict raged on.

In the latest violence, an air strike on a high school in a rebel-held city of northern Syria killed 12 people, most of them students, the Syrian Observatory for Human Rights said.

And ahead of the start of the disarmament mission on Tuesday, UN chemical arms investigators left their Damascus hotel to carry out a new inspection on the ground.

The United Nations has said the experts, who have been in Syria since last Wednesday, are investigating seven alleged chemical weapons attacks and expect to wind up their work on Monday.

They hope to have a comprehensive report ready "by late October".

The UN mission is separate from a team of Organisation for the Prohibition of Chemical Weapons inspectors due to start work the next day to destroy Syria's chemical arsenal.

The mission is also still investigating an August 21 attack in Damascus suburbs that coincided with an earlier visit to Syria. The experts have already confirmed the use of the nerve agent sarin in the area.

Both the Syrian regime and the rebels seeking its overthrow have repeatedly accused each other of using chemical weapons.

But in the wake of the August 21 attack, which allegedly left hundreds dead, the United States threatened military action against the Syrian regime, which it accuses of responsibility.

The regime denies involvement but agreed soon afterwards to relinquish its chemical arsenal, averting a strike, under a US-Russian deal which was enshrined in a landmark UN Security Council resolution on Friday.

UN chief Ban Ki-moon on Saturday held his first meeting with Syria's opposition National Coalition chief to press for a peace conference aimed at ending the 30-month conflict that has cost more than 110,000 lives.

Coalition leader Ahmad Jarba told Ban the opposition group was ready to send a delegation to a conference, according to UN spokesman Martin Nesirky.

Ban has set mid-November as the target date for convening the conference which aims to build on the work of a previous one held in in June 2012.

In his talks with Jarba, Ban "stressed the importance of embarking on a serious dialogue as soon as possible as well as the need to ensure accountability for war crimes," his spokesman said.

Syrian Foreign Minister Walid Muallem, meanwhile, signalled there could be no talk of President Bashar al-Assad's departure, as the opposition and Western governments have demanded.

"There can be no discussion of the future of President Assad. It is in the constitution," he said at UN headquarters in New York.

As fighting between Assad's forces and rebels raged, his air force "bombed a technical high school in the city of Raqa, killing 12 people, most of them children under 18," the Syrian Observatory said.

The Britain-based group posted video footage showing mangled bodies, one of them lying under schoolbooks. Its authenticity could not immediately be verified.

Raqa, on the Euphrates valley 160 kilometres east of the main northern city of Aleppo, is the only provincial capital entirely in rebel hands.

Captured from government forces on March 6, the city is now largely controlled by Al-Qaeda loyalists of the Islamic State of Iraq and the Levant.

The air strike came after rebels launched an overnight attack on army positions in Nasseriya al-Qalamun, north of Damascus, killing at least 19 soldiers and wounding 60, the Observatory said.

In one of the biggest and most dangerous operations ever staged, international chemical weapon troubleshooters are due to enter war-torn Syria on Tuesday.

With more than 1,000 tonnes of sarin, mustard gas and other banned horror chemicals stocked across the country, the United Nations and the global chemical weapons watchdog have launched an urgent appeal for scarce experts to join the mission.

Final details of the US-Russia plan to dispose of stockpiles at an estimated 45 sites by a target date of mid-2014 have still not been agreed, UN diplomats said.

The Syrian government has submitted an initial list of chemical sites and weapons. It has until Friday to send more information to the Organisation for the Prohibition of Chemical Weapons tasked with the cleanup.