A Danish ship carrying some of Syria's most dangerous chemical agents has begun transferring them onto an American cargo vessel equipped with special gear to neutralise them at sea.

The pier-side handover in southern Italy comes after a series of delays in the internationally-backed plan to destroy the Damascus government's stock of toxic arms.

Denmark's Ark Future, which had brought the agents from Syria, started offloading the first containers in the port of Gioia Tauro, said police and officials from the Organisation for the Prohibition of Chemical Weapons (OPCW).

They were taken to the converted US container ship Cape Ray which will sail them into the middle of the Mediterranean over the next few days and start transforming them into a much less poisonous soup of chemicals, ready for disposal back on land, the officials said.

Experts have said the process could take between 60 to 90 days, depending on the weather.

The Damascus government, fighting rebels for more than three years, agreed to hand over its stockpile, which includes precursors for deadly nerve agent sarin gas, under a deal backed by Washington and Moscow in September.

The agreement averted US military strikes after hundreds of people were killed in a sarin gas attack in the outskirts of Syria's capital, Damascus.

Last month, Syria handed over the remaining 8 percent of the 1,300 tonnes it declared to the OPCW, which is monitoring the process.

The Syrian government had said fighting delayed the transfer of the last parts of its stockpile. The Cape Ray had been docked in southern Spain for weeks.