A military mission to secure Syria's chemical arsenal would require a large ground force and pose huge risks, with the outcome hinging on the quality of Western intelligence, experts say.

With the Syrian regime suspected of using chemical agents against rebels, US and Western military commanders are planning for a possible worst-case scenario in which an international force would move in to neutralize the lethal weapons.

Any attempt to seize control of chemical agents in Syria would depend on the intelligence gathered by foreign spy services, which have struggled at times to track the Damascus regime's stockpiles.

"The first thing is you have to know where they are. Where are the weapons stored and where are the production sites and production-related sites?" said David Kay, the former UN weapons inspector who led the Iraq Study Group.

"It seems obvious but it's not easy," Kay, now a senior fellow at the Potomac Institute for Policy Studies, told AFP.

Syria is believed to have hundreds of tons of chemical agents such as sarin and VX as well as mustard gas, but the precise details of its arsenal remain unknown.

To take control of the weapons, the United States or its allies would have to send in boots on the ground, including teams of technical experts, special forces units and a large contingent -- likely tens of thousands -- of conventional troops to seize and guard chemical sites, analysts said.

"Even under the best of circumstances, it's a highly manpower intensive effort because you have to -- 24/7 -- surround the sites, and ensure that others are not entering," Kay said.

The intervention force would have to launch bombing raids to take out air defenses as a preliminary step and would also likely need to target some depots that could not be seized by a ground force.

"There are probably some facilities that will be beyond our reach that we will have to use air strikes to deal with," said Michael Eisenstadt, a senior fellow at the Washington Institute.

At sites seized by ground forces, troops would need to stand guard while weapons experts -- clad in protective clothing and using special medical equipment -- document and neutralize the chemical agents and munitions.

The weapons experts would likely have to destroy the nerve agents or lethal gas on site, as transporting them would be too risky, according to Kay.

As special incinerators would not be available, the lethal agents likely would be placed in a lime pit and broken down with other chemicals, reducing toxicity to a level equivalent to industrial waste.

The painstaking process would take "weeks or months even," said Eisenstadt.

The intervention force would also need to destroy missiles, aerial bombs and artillery shells used to deliver chemical agents, which would likely require a wave of air strikes.

"You don't destroy the chemical weapons themselves but all the ways in which they can be delivered to the population," said Elizabeth O'Bagy of the Institute for the Study of War.

Part of the arsenal could be "entombed" by bombing designed to collapse and seal bunkers with rubble, Eisenstadt said.

"We've developed a series of munitions that could be used for these kind of missions," he said.

The US military has a bomb, the BLU-119/B, specifically designed to incinerate chemical agents, though the Pentagon has never disclosed how it has fared in tests.

Air raids also could be used to prevent an adversary from accessing a chemical arms site, but any attempt to bomb from the air would be fraught with danger.

Flawed intelligence or a single mistake in an air strike could cause a disastrous release of chemical agents that would threaten the very civilians that the United States and its allies hope to protect.

Trying to secure the chemical stockpiles would be an unprecedented undertaking, so filled with peril and uncertainty that some have called it an impossible mission.

Even if an operation went mostly as planned, parts of the arsenal would likely fall into the wrong hands, analysts said.

It would be "very tough," said Kay. "There are no good options."