The most crucial part of the operation to eliminate Syria's chemical weapons has been completed, the organisation charged with their destruction has said.

The Organisation for the Prohibition for Chemical Weapons said on Tuesday that 581 tonnes of a precursor for sarin gas and 19.8 tonnes of mustard gas had been destroyed aboard a US ship, the Cape Ray.

The toxicity of the chemicals was reduced by 99.9 percent, and the resulting liquid will now be transported to Finland and Germany for final disposal.

OPCW experts ensured that none of the chemicals escaped into the sea, the agency's director-general Ahmet Uzumcu said in a statement.

The destruction of Syria's chemical weapons was first ordered last year by the UN following a series of deadly chemical attacks by the government on opposition-held areas during the country's more than three-year-long conflict.

One of the largest attacks was reported in the Damascus suburb of Ghouta last August, when up to 1,000 people, the vast majority of whom were civilians, were killed when the rebel-held area was hit by rockets containing sarin.

The UN found that the sarin used was of a quality found only in Syrian regime stockpiles, but did not specifically blame one side for the attack.

The Syrian government was then forced to dismantle its chemical weapons programme, however, there are still widespread allegations that president Bashar al-Asad did not disclose all of its chemical weapons sites to the UN and therefore maintains stockpiles.

Source: Agencies