* Offshore option prompted by Albanian refusal to host facility
* Official says floating destruction site is technically feasible
* Floating facility would be unprecedented in scale, experts say
* Lesser operations have been conducted offshore
* Environmental concerns about processing toxic waste at sea
Syria's chemical weapons could be processed and destroyed out at sea, say sources familiar with discussions at the international body in charge of eliminating the toxic arsenal.
Four days after Albania rejected a U.S. request that it host a weapons decommissioning plant, Western diplomats and an official of the Organisation for the Prohibition of Chemical Weapons at The Hague told Reuters the OPCW was studying whether it might carry out the work at sea, on a ship or offshore rig.
Confirming the discussion, the OPCW official stressed there had been no decision: "The only thing known at this time is that this is technically feasible," the official said on Tuesday.
While other states, notably Japan, have dealt with chemical weapons at sea, mounting such a large and complex operation afloat would be unprecedented, independent experts said.
But given the equally daunting challenge of neutralising over 1,000 tonnes of material in the middle of a civil war, and the reluctance of governments like Albania to defy popular protests against hosting any facility, it is being considered.
"There are discussions about destroying it on a ship," one U.S. official told Reuters.
Syrian President Bashar al-Assad agreed to join a global ban on chemical weapons after Washington threatened air strikes following a major sarin gas attack on rebel-held territory in August, for which the Damascus government blamed its enemies.
OPCW inspectors have checked Syria's declared 1,300 tonnes of sarin, mustard gas and other agents and the organisation decided last week that most of the deadliest material should be shipped abroad by the end of the year and destroyed by mid-2014.
While battles for control of the highway from the capital to the Mediterranean port of Latakia have raised questions over the trucking of the chemicals to the coast, the Albanian refusal on Friday took negotiators by surprise, sources said, and prompted a radical shift in thinking to keep the plan on schedule.
Ralf Trapp, an independent chemical disarmament specialist, said of the offshore decommissioning suggestion: "It had to come up as a option at some point in time, given the circumstances.
He added: "Technically it can be done, and in fact at a small scale it has been done."
Japan destroyed hundreds of chemical bombs at an offshore facility several years ago. And Trapp said setting up a disposal plant on a floating platform might not differ greatly from the Pacific atoll where the United States destroyed much of its chemical arsenal through the 1990s.
Trapp said Syria's stockpile would require more complex treatment than the World War Two bombs that Japan found on the seabed, raised and destroyed off the port of Kanda from 2004-06.
The Japanese munitions, as a finished product, did not produce liquid waste, he said. By contrast, much of Syria's stockpile is of bulk "precursor" materials that were stored in order to manufacture weapons at a later stage. Burning these, or neutralising them with other chemicals in a process known as hydrolysis, would produce large amounts of toxic fluids.
"If you use hydrolysis or incineration, there will be liquid waste," Trapp said. "So there will be problems with regard to environmental pollution that need to be addressed."
Countries around the Mediterranean might not relish the prospect of such an operation, though shipping the Syrian material further afield could also pose difficulties.
Siting a facility close to shore could risk the kind of demonstrations in Tirana that forced Albania's government to change tack. Further out at sea could pose other problems, such as providing a rapid response to emergencies.
Trapp said that a "large floating platform at sea would not be fundamentally different" from the now dismantled U.S. chemical weapons destruction facility at Johnston Atoll in the North Pacific: "There are many technical and legal challenges," he said. "But it may be an alternative worthwhile considering."