More than 90 percent of Syria's chemical weapons material has been removed and destroyed, said Sigrid Kaag, the head of the Joint Mission of the Organization for the Prohibition of Chemical Weapons (OPCW) and the United Nations Thursday.

"I strongly encourage the Syrian authorities to conclude the removal operations as part of their efforts to achieve the 30 June 2014 deadline," said Kaag, the special coordinator of the Joint Mission, in a statement.

The removal of the most critical material for destruction began in early January, in line with an agreement brokered by Russia and the United States under which Syria renounced its chemical weapons material and joined the 1992 Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons.

In addition to the removal operations, the Syrian authorities have destroyed buildings, equipment and empty mustard gas containers, and decontaminated other containers in a number of chemical weapons storage and production sites, said Kaag. "A majority of these sites are now closed."

The UN Security Council unanimously adopted a resolution in September last year to rid war-torn Syria of its chemical weapons.