UNITED NATIONS (AFP) - International chemical weapon troubleshooters will enter war-torn Syria on Tuesday to start one of the biggest and most dangerous disarmament operations ever staged.

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.

Applicants must be ready to face mortal risks and an impossible deadline.

UN leader Ban Ki Moon called the operation "daunting" after the UN Security Council voted on Friday to eliminate President Bashar al-Assad's chemical arms.

The mission by the Organization for the Prohibition of Chemical Weapons (OPCW), which polices the 1993 Chemical Weapons Convention, will run in parallel to a UN investigation into a huge sarin gas attack in Damascus in August and other suspected attacks.

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

Cleanups of chemical nasties have been staged in Iraq and Libya, but never in the middle of a raging war.

The UN says the Syrian conflict has already left more than 100,000 dead.

Experts say the OPCW will need up to 200 inspectors for the Syria force. It currently has less than half that number who already have a heavy regular workload. The watchdog has had to appeal to the major powers to send scientists.

Those who go will become a new target in the 30-month-old conflict and the strife means the noxious potions will have to be moved out of Syria to be destroyed.

The US-Russia plan sets a target date of mid-2014 for completion, but few people believe it is achievable.

"No operation this big has been carried out before and certainly not in a war," said Dina Esfandiary, a disarmament specialist with the International Institute for Strategic Studies in London.

The Syrian government has declared one list of chemical sites and weapons.

It has until Friday to send more information to the OPCW. "The first obstacle is it is very difficult for us to know for certain whether Assad has declared all his stockpiles," Esfandiary told AFP.

"The inspectors will then become perfect targets in a situation of civil war. Anyone who wants to derail the process - and I am sure a lot of people will want to do that - will be able to target them as they visit the various sites." The UN investigators have already been the target of snipers in Damascus.

Hamish de Bretton-Gordon, a consultant and former commanding officer of Britain's chemical and nuclear defences force, said that destroying the chemicals in Syria is not an option in the middle of a war.

Finished chemical bombs are normally destroyed in furnaces that would be impossible to build in Syria.

In Iraq, experts sometimes just dug holes in the desert and put in fuel and a detonator to incinerate the chemicals. Syria's dangers mean that even those measures are not possible.

"The ideal plan would be for the Syrians to deliver chemical weapons to the border, where the UN and the major powers take over to get rid of them," Bretton-Gordon told AFP.

He estimated that it would take about 200 experts working flat out for six months to get all the chemicals into international hands.

"It is going to take a lot longer to destroy them," said the expert, whose company is training doctors and nurses in Syria on dealing with chemical warfare. "It could take a couple of years." The United States and Russia have embarked on voluntary programs to destroy their chemical arsenals.

But they have both have asked the OPCW for a deadline extension because the task is so complicated.

Both countries have the furnaces to dispose of the chemicals, but Russia has indicated it is reluctant to accept Syrian arms.

The cost will also be huge. Assad has said it will take one year and one billion dollars to get rid of his weapons.

Experts warn this will be just the cost of getting the chemicals out of Syria. "In resource terms, it is going to be huge," said Bretton-Gordon.

A UN diplomat likened the operation to a "minefield." "You defuse one bomb and there is another waiting," the diplomat added.

Esfandiary noted it was likely the scientists would fail to meet the deadline.

"But even if they are able to degrade a portion of Syria's weapons, then it is worth pursuing," she said.