The head of the mission overseeing the destruction of Syria's chemical arsenal urged Damascus on Wednesday to urgently hand over its remaining agents, and pressed countries with influence to intervene.

Under a deal backed by the United Nations and brokered by the United States and Russia after Washington threatened air strikes against Syrian government targets, the weapons were to be destroyed by June 30.

But 7.2 percent of Syria's declared chemical agents -- 16 containers packed and ready to go -- remain outside Damascus, Sigrid Kaag, the UN official overseeing the process, told AFP.

Gripped by a civil war now in its fourth year, Syria says it is currently too dangerous on the ground to transport the chemicals safely by road to the port of Latakia.

Danish and Norwegian ships are waiting to take the chemicals to a US ship for a destruction process that lasts around 60 days.

"We call on all member states to exercise their influence to ensure the immediate removal of the remaining chemicals," said Kaag, head of the joint UN and Organization for the Prohibition of Chemical Weapons (OPCW) mission to disassemble the weapons.

"The urgency, the time, the pressure to remove the remaining 7.2 percent is very, very critical," she told reporters after briefing the Security Council behind closed doors.

- Mission ending in September? -

Western diplomats say a key question is how long the mission will go into overtime.

Kaag said in an interview that she hoped it could be wrapped up by the end of September.

A staff of 110 people in Syria will be slashed to fewer than 50, including 15-20 international personnel, after June 30.

"Our hope is that by the end of September everything will have been concluded," Kaag told AFP.

Technical visits to inspect Syria's progress every two months could then be managed from The Hague.

But Kaag was upbeat earlier this year about meeting the June deadline. Syria has now missed a string of target dates.

She admitted there was "no magic wand" to force compliance, but said senior Syrian officials and "important member states" had given assurances that it would happen "fairly shortly."

Security concern "doesn't mean, however, that there's not a tremendous urgency to get the job done," Kaag warned.

Western diplomats, pressing for a UN resolution to enforce humanitarian corridors in Syria, are waiting for a separate report on the regime's alleged ongoing use of chlorine gas.

Humanitarian chief Valerie Amos said that there had been a marked increase in violence during the conflict, including "poisonous gases allegedly used against civilians."

France and the United States allege government forces may have unleashed industrial chemicals on a rebel-held village in May.

Syria did not have to declare its stockpile of chlorine -- a weak toxic agent -- as part of the disarmament deal as it is widely used for commercial and domestic purposes.

Kaag refused to be drawn on the issue of chlorine as an OPCW fact-finding mission is a separate operation.

The mother of four, a veteran Dutch diplomat who commands huge respect, acknowledged the extraordinary complexity of her mission as well as the risks braved by her staff on the ground.

"There are days when 12-13 mortars hit the area around our working premises which is also where we live," she told AFP.

"The concern I think in this mission, particularly for us, has always been that you don't want to be at the wrong place at the wrong time. but who can predict."

Amos said the 6.5 million Syrians displaced by the war account for 20 percent of the total number of people internally displaced by conflict across the world.

Asked what she would say to President Bashar al-Assad, re-elected in controversial polls on Tuesday, Amos said: "I would say to him put the people of Syria first."

In an ocean of bad news from Syria, the weapons dismantlement sticks out as a relative success and Kaag is reportedly a possible successor to international mediator Lakhdar Brahimi.

But she refused to be drawn on her next job.

"I'm very focused on this mission to wrap it up properly and then we'll see," Kaag said.