An unprecedented effort to destroy Syria's most dangerous chemical agents aboard a US naval ship in the Mediterranean is making progress, the Pentagon said Friday.

The American specialists on the MV Cape Ray so far have neutralized about a quarter of the chemical DF, a precursor used to make lethal Sarin gas, officials said.

"As of this morning, the crew has neutralized over 25 percent of the Sarin precursor," Pentagon spokesman Colonel Steven Warren told reporters.

Out of about 581 metric tonnes of DF on the ship, roughly 147 tonnes of the precursor have been destroyed so far, officials said.

"The crew aboard the MV Cape Ray continue their work to neutralize materials from Syria's declared chemical stockpile," Warren said.

The ship also is carrying more than 19 tonnes of mustard gas, which has not yet been neutralized.

Other chemical agents from Syria are being destroyed on land at special facilities in Britain, Finland and the United States.

Overall, about 31 percent, or 411 tonnes, of various chemical agents have been neutralized so far in the multinational effort, according to the Organisation for the Prohibition of Chemical Weapons (OPCW), the international agency overseeing the work.

After a global outcry over deadly chemical attacks in a Damascus suburb last year, the Syrian regime of President Bashar al-Assad agreed to an international plan to destroy its chemical weapons stockpile.

With no country ready to accept shipments of the most lethal chemical agents, the United States proposed destroying the chemicals at sea using two "portable" hydrolysis units.

The process calls for mixing the chemicals in sealed containers with thousands of gallons of hot water along with sodium hydroxide or other "reagents" that help break down the toxicity of the chemical.

The resulting effluent, which is the equivalent to industrial waste, will be transported to plants on land for disposal.

Officials say none of the chemicals or waste will be dumped at sea.

US officials have kept secret the precise location of the MV Cape Ray in the Mediterranean Sea.

Activists on the Greek island of Crete launched a seaborne protest on Friday against the destruction work, the state-run ANA agency reported.

The campaigners plan to reach an area southwest of Crete where they believe the breakdown of the chemical agents is being carried out.