The head of an international mission to Syria charged with destroying the country's chemical weapons has called on President Bashar Assad's government to ensure it meets a deadline to destroy all its toxic chemicals.

Sigrid Kaag of the Organisation for the Prohibition of Chemical Weapons told reporters in Damascus that 92.5% of Syria's chemical materials had been removed from the country and destroyed.

She described it as "significant progress," though she said the government needs to ensure the remaining materials are eradicated by the end of the month.

"It's important that the last push is made toward full completion," Ms Kaag said.

Syria missed an April 13 deadline to destroy all its chemical weapons in accessible locations. International experts say that could have a serious impact on making a June 30 deadline for the removal of all its chemical weapons.

"An important (achievement) has been made in permanently closing down production facilities," Ms Kaag said, adding it came in "a very short period of time and under difficult and challenging security conditions".

However, experts have not had access to 12 chemical weapons production facilities yet, Ms Kaag said.

The international community aims to remove and destroy 1,300 metric tons of chemicals Syria stockpiled to turn into poison gas and nerve agents.

The effort was sparked by an August 21 chemical weapons attack near Damascus that killed hundreds of people. The attacks was blamed on Assad's government and brought the United States to the brink of military intervention in Syria. Damascus denied involvement.

In recent weeks, activists accused government forces of attacking rebel-held areas with poisonous chlorine gas, leaving men, women and children coughing, choking and gasping for breath.

Syria flatly denied the allegations, and they have yet to be confirmed by any foreign country or international organisation. But if true, they highlight the limitations of the global effort to rid Syria of toxic material.