(Adds interview with Kaag)
By Louis Charbonneau and Michelle Nichols
UNITED NATIONS, Feb 6 (Reuters) - The head of an international chemical weapons mission in Syria said on Thursday she did not believe the Syrian government was intentionally delaying the removal of its arsenal, but that accelerated cooperation was vital to meet a June 30 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 U.N. Security Council behind closed doors.
Syria failed to meet an OPCW deadline of Feb. 5 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 June 30.
"We think the deadline of 30th June can be met. All equipment and requirements are in country," Kaag said after briefing the council, which adopted in September a legally binding resolution demanding Syria's chemicals be eradicated.
"Intermediate milestones (like Feb. 5) ideally should have been met. They have not been met, there are delays," she said.
But when asked if the Syrian government was deliberately stalling on its responsibility to transfer all its chemicals to the port of Latakia for shipment abroad, Kaag said: "No, I don't think so ... Delays are not insurmountable. Delays have a reason, there's a rationale, there's a context."
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 U.S. threat of military strikes, which was averted by Assad's pledge to give up chemical arms.
"The Assad regime must immediately take the necessary steps to fulfill its obligations," U.S. ambassador to the United Nations, Samantha Power, told reporters after 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."
Kaag's briefing to the council was crucial because if the final deadline of June 30 is missed, the Security Council will likely have to consider whether Syria was deliberately not complying with the internationally agreed plan.
Power warned that delays by Assad's government were "encouraging heightened risks that these weapons will be used again, by regime elements, or will fall into the hands of terrorists."
Kaag said mustard gas made up only 1.5 percent of Syria's declared chemicals, though it is one of the most worrisome parts of the program. "Once that gets removed for onward destruction, a significant harmful agent is out of country," she said.
In an interview with Reuters, however, Kaag said it was possible for the Syrian government to complete the total elimination of its chemical weapons program ahead of June 30.
Kaag said her mission would like "volume-based predictable movements" of chemicals in accordance with a schedule that her mission will keep confidential for obvious security reasons.
"That's what we expect, that regular deliveries (of chemicals) will pick up," she said.
While the international media focus has been on the slow shipment of chemicals out of the country, Kaag said destruction of chemical weapons-related material inside Syria has continued.
Lithuania's U.N. ambassador, Raimonda Murmokaite, president of the Security Council for February, said the council "noted growing concern about the slow pace of the removal of chemical weapons from the territory of Syria."
She said the council called on Syria "to expedite actions to meet its obligation to transport in a systematic and sufficiently accelerated manner all relevant chemicals to Latakia for removal from Syrian territory."
Britain's U.N. ambassador, Mark Lyall Grant, told reporters before 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, Assad's main diplomatic protector in the Security Council, suggested on Wednesday 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 U.N. envoy Vitaly Churkin told reporters. "The Syrian government has done a couple of runs already."
"They are working very hard to be as technically prepared as humanly possible and still complaining that they are missing some technical support which they require but we believe that generally the main components are there," he said.
U.N. Secretary-General Ban Ki-moon said in a report last month that Syria has enough equipment to transfer the chemicals out of the country. Syria has also blamed the delays on security concerns and the weather. (Editing by Nick Zieminski and Mohammad Zargham)