Danish ship Ark Futura arrives in the port of Gioia Tauro, southern Italy, on July 2, 2014 for the transfer of chemical weapons from Syria to be destroyed. -AFP

AFP/Gioia Tauro

The operation to transfer Syrian chemical weapons from a Danish freighter to a US military ship ahead of their destruction at sea kicked off on Wednesday, expected to last up to 20 hours.

With the vessels moored stern-to-stern within a wide safety zone set up around the port of Gioia Tauro in Italy, the first of 78 containers was taken off the Ark Futura by crane and manoeuvred onto the MV Cape Ray by a vast climbing platform.

The first three containers to be transferred hold a total of 20 tonnes of mustard gas, while the remaining 75 contain among other things the raw materials for Sarin nerve gas, according to Italian media reports.

Once the chemical agents have been safely transferred, they will be destroyed in international waters in the final phase of a programme to rid Syria of its chemical arms stockpile.

"Proud of Italy's contribution to international security, (and) a transparent operation which is environmentally safe," Italy's Environment Minister Gian Luca Galletti said on Twitter as the transfer began.

His comments came after concern among local inhabitants over the possible health fallout from the operation, with protesters dubbing the Ark Futura "the poison ship".

"This is not a routine operation, it's a military operation and we are very worried," trade unionist Domenico Macri told AFP television.

"We have never carried out this type of operation in Gioia Tauro before. If there's an accident, a container breaks or falls, the substances which would come out could do serious damage," he said.

The port has stepped up security for the transfer, sealing off access roads and barring entry to any non-authorised people, while a military helicopter flew overhead as the Cape Ray arrived before dawn.

Inspectors from the Organisation for the Prohibition of Chemical Weapons (OPAC) boarded the Ark Futura to check the cargo before the transfer began.

Once the Cape Ray moves back out into international waters, the process to destroy the agents and materials is expected to take between 45 and 90 days.

The US vessel has been equipped with two Field Deployable Hydrolysis Systems -- portable treatment plants capable of "neutralising" the most dangerous Syrian chemical agents.

The process should destroy more than 99 percent of the chemicals, reducing the lethal agents into a sludge similar to low-level hazardous industrial waste, which will then be disposed of by private waste treatment facilities.

Syria shipped out its stockpile of chemical weapons under the terms of a UN-backed and US-Russia brokered agreement to head off Western air strikes against the regime last year.