Syria still holds around 8% of its declared chemical weapons material, despite the deadline for it to be handed over expiring this morning, the task force overseeing the operation has said.

Sigrid Kaag, head of the joint Organisation for the Prohibition of Chemical Weapons-UN team overseeing the destruction of the arsenalm said "7.5-8%" of Syria's chemical weapons material remained in-country at "one particular site"

"However, 92.5% of chemical weapons material removed or destroyed is significant progress," she said.

"We also however need to... ensure the remaining 7.5-8% of the chemical weapons material is also removed and destroyed."

Of that amount, 6.5% would be removed from Syria, she said.

"A small percentage is to be destroyed, regardless, in-country. That can be done. It's a matter of accessing the site," she added.

Ms Kaag, speaking at a press conference in Damascus, praised the "very constructive cooperation" of the Syrian government under difficult security circumstances.

"We are mindful that security has many faces and can be very challenging. However, as a state party [to the Chemical Weapons Convention], Syria also needs to acquit itself of its commitments," she said.

Under a US-Russian deal negotiated last year, Syria signed up to the Chemical Weapons Convention and agreed to hand over its entire chemical weapons arsenal by 30 June of this year.

Ms Kaag said she was "hopeful" that the deadline could be met, despite Syria's failure to meet today’s deadline for removing or destroying all its chemical weapons material.

In addition to the remaining material, Syria also needs to complete the closure or destruction of production and storage facilities.

There is a dispute over whether Syria will have to destroy 12 remaining chemical weapons production sites.

Syria wants to seal the sites, which it says have already been rendered unusable, but Western countries want them completely destroyed, fearing that they may be used in the future.

"Success is around the corner and this last push is very much needed," Ms Kaag said.

"The 30 June deadline is around the corner... and we are hopeful that this is possible and will be met."

Syria's government agreed to turn over its chemical arsenal last year as the US threatened military action in response to a deadly chemical weapons attack outside Damascus that reportedly killed 1,400 people.

Activists and much of the international community blamed the attack on the regime, although it denied responsibility.

Syria has missed several deadlines on track towards the 30 June completion date, blaming the delays on the security situation in the country.

It declared around 700 tonnes of the most dangerous chemicals and 500 tonnes of less dangerous precursor chemicals.

There are also questions over alleged attacks involving the use of chlorine gas in Syria in recent weeks.

The regime says a jihadist group, Al-Nusra Front, used the chemical in an attack on the rebel-held town of Kafr Zita, but the opposition blames the government.

The competing claims have raised concern in the United Nations, with some Security Council members calling for a probe into the allegations.