THE HAGUE — Syria is expected to hand over a detailed plan for the destruction of its chemical arsenal in the next 24 hours, days ahead of deadline, the world's watchdog said Wednesday.

The detailed plan is the next step for Damascus under the terms of a U.S.-Russian deal to head off military strikes on Syria agreed last month which calls for all its chemicals to be destroyed by mid-2014.

"We expect Syria's initial declaration of its chemical weapons program within the next 24 hours," spokesman Michael Luhan told reporters in The Hague, where the Organization for the Prohibition of Chemical Weapons (OPCW)  is based.

President Bashar Assad's regime has already handed over an inventory of its chemicals, weapons, and facilities, and international inspectors are already busy inspecting and destroying them.

Syria has until October 27 to submit its "initial" plan of how to destroy its weapons, in accordance with Article III of the Chemical Weapons Convention.

A joint OPCW-United Nations mission has now checked 18 of 23 declared sites in Syria, destroying production equipment at almost all of them, Luhan said.

Damascus is required to submit its declaration by Sunday in accordance with deadlines set by the OPCW's Executive Council and enforced by a U.N. Security Council resolution.

The OPCW's Executive Council will use the Syrian declaration to decide by November 15 on "destruction milestones" for Syria's arsenal.

With the first stage of the OPCW's work in Syria nearing completion, Luhan said that some inspectors would be leaving Damascus next week, temporarily reducing the watchdog's staff there from 28 to 15.

The combined OPCW-U.N. team in Syria is currently around 60-strong.

"We're reaching the end of this first stage, during which we needed to verify the chemical warfare program by visiting all the disclosed sites," Luhan said.

A combined OPCW-U.N. logistics team has also been deployed to nearby Cyprus, he added.

Mission coordinator Sigrid Kaag said in Damascus on Tuesday that Syria was cooperating with the task of destroying its chemical stockpile.

The U.N. Security Council last month passed a resolution backed by the OPCW agreement to destroy the arsenal after hundreds of people were killed in sarin gas attacks on the outskirts of Damascus on August 21.

The United States threatened military strikes, but then reached a deal with key Damascus backer Russia.