Destroying Syria's chemical weapons under a Russian plan would be "immensely difficult" and may do little to end the conflict there, the International Institute for Strategic Studies (IISS) said on Thursday.
Russia and Iran would have to play a role in dismantling the chemical arms stockpiled by the regime of President Bashar al-Assad in a process that would take years, said experts from the London-based think-tank.
"There has never been a situation where the international community has attempted to secure, seize and destroy weapons of mass destruction during an ongoing conflict," IISS proliferation expert Mark Fitzpatrick told a news conference at the launch of the group's annual review of world affairs.
"The best case was in Iraq and even there it took many months to assemble teams and years to destroy the arsenal. In Libya, it has been many years and still not all the mustard gas has been destroyed," he said.
"So obviously it's immensely difficult. The US Department of Defense has estimated that 75,000 troops would be required to secure the chemical weapons" in Syria.
Moscow and Washington were holding high-stakes talks on Thursday on Russia's plan for Syria to eliminate its chemical weapons in order to avoid air strikes threatened by US President Barack Obama.
Assad confirmed in a Russian TV interview Thursday that Syria would hand over its chemical weapons.
The United States, France and Britain want the dismantling of Assad's arsenal to take place under a UN Security Council resolution backed by the threat of force, but Russia opposes any mention of military action.
IISS Director-General John Chipman said the international community should not let the search for peace in Syria get bogged down in the narrower issue of chemical weapons.
"Whether this diplomacy over chemical weapons use accelerates the diplomacy for a resolution of the Syrian civil war is quite another matter," he said.
"It will be a lengthy and disputed process to put Syrian chemical weapons under international control; that effort should be used to encourage, not dissipate, efforts to solve the actual conflict."
Emile Hokayem, a Middle East analyst at IISS, said there was "sense of rejection and abandonment" among the Syrian rebels fighting Assad.
"I think that in the coming months in fact the fighting is going to increase massively and we will probably see even more massacres just because there is a sense right now that there is no outside help coming so it's free for all," he said.
There are also concerns about how to keep Syria's chemical weapons out of the hands of extremist groups among the rebels.
Asked how to ensure that, Fitzpatrick, the proliferation expert -- who used to work for the US State Department -- said that "all states that have a stake in Syria might take responsibility... and I would of course include Russia and Iran".