The international chemical weapons watchdog says it is studying details submitted by Syria about its arsenal of poison gas and nerve agents.

A spokesman for the Organisation for the Prohibition of Chemical Weapons said it was expecting more details from the government in Damascus ahead of the deadline later today for complete disclosure of its weapons and research sites.

Syria needs to fill in gaps on details it handed into The Hague yesterday by next week as part of a rapid disarmament operation that may avert US airstrikes.

An OPCW spokeswoman said: "We have received part of the verification and we expect more."

She did not say what was missing from a document one UN diplomat described as "quite long".

The OPCW'S 41-member Executive Council is due to meet early next week to review Syria's inventory and to agree on implementing last week's US-Russian deal to eliminate the entire arsenal in nine months.

The timetable was laid down by US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov a week ago in Geneva when they set aside sharp differences over Syria to agree on a plan to deprive Bashar al-Assad of chemical weapons and so remove the immediate threat from Washington of launching military action.

Security experts say it has about 1,000 tonnes of mustard gas, VX and sarin – the nerve agent UN inspectors found after hundreds were killed by poison following missile strikes on rebel-held areas on 21 August.

One Western diplomat warned that a failure by Mr Assad to account for all the suspected stockpile would cause world powers to seek action at the UN Security Council to force him to.

The US State Department said it was studying the material: "Today was a step that we're looking for in terms of an initial document," said spokeswoman Marie Harf.

The Syrian government has accepted the plan and has already sought to join the OPCW.

Once the OPCW executive has voted to follow the Lavrov-Kerry plan in a meeting expected early next week, the Security Council is due to give its endorsement of the arrangements - marking a rare consensus after two years of East-West deadlock over Syria.