The head of an international chemical weapons mission in Syria has said she did not believe the Syrian government was intentionally delaying the removal of its arsenal.

However, she added that accelerated cooperation was vital to meet a 30 June deadline.

Sigrid Kaag, head of the United Nations and Organization for the Prohibition of Chemical Weapons (OPCW) mission charged with overseeing the destruction of Syria's chemicals, briefed the 15-member UN Security Council behind closed doors.

Syria failed to meet an OPCW deadline of 5 February to move all of its declared chemical substances and precursors out of the country.

The final deadline under the OPCW plan is for all of Syria's declared chemical materials to be destroyed by 30 June.

"We think the deadline of 30 June can be met. All equipment and requirements are in country," Ms Kaag said after briefing the council, which adopted in September a legally binding resolution demanding Syria's chemicals be eradicated.

"Intermediate milestones (like 5 February) ideally should have been met. They have not been met, there are delays," she said.

Syrian President Bashar al-Assad agreed to destroy his chemical weapons following global outrage over a sarin gas attack in August - the world's deadliest chemical attack in 25 years.

That attack sparked a US threat of military strikes, which was averted by Mr Assad's pledge to give up chemical arms.

"The Assad regime must immediately take the necessary steps to fulfil its obligations," US ambassador to the UN, Samantha Power, told reporters after Ms Kaag's briefing.

"We know the regime has the ability to move these weapons and materials because they have moved them multiple times over the course of this conflict," she said.

"It is time for the Assad government to stop its foot-dragging, establish a transportation plan and stick to it."

Britain's UN ambassador, Mark Lyall Grant, told reporters before Ms Kaag's briefing that "the time for excuses has run out".

"Everything is in place and there needs to be proper arrangements for accelerating the process of getting the chemicals out of Syria," he said.

"The international community has bent over backwards."

Russia, Mr Assad's main diplomatic protector in the Security Council, suggested yesterday that the Syrian government appeared to have the situation under control.

"They're now in a good place as far as this program is concerned," Russian UN envoy Vitaly Churkin told reporters."The Syrian government has done a couple of runs already."