Insecurity in war-torn Syria is hampering the final stages of work to dismantle its chemical weapons arsenal, the UN official overseeing the task told reporters on Thursday.

Speaking to reporters following a closed door meeting of the UN Security Council, Sigrid Kaag said 92 percent of Syria's illicit chemical stockpiles have been destroyed, as a June 30 deadline approaches.

But dangerous conditions on the ground have made it impossible to access the remaining chemical weapons containers.

"There is concern that the remaining eight percent is currently inaccessible due to the security conditions in the country," said Kaag, special coordinator for a joint mission by the United Nations and the Organization for the Prohibition of Chemical Weapons (OPCW) to disassemble the weapons.

"We do need that final push to achieve 100 percent," she said.

Work on removing and destroying the material began in January, in accordance with a deal brokered by the United States and Russia.

Just 16 containers of deadly material remain, Kaag said, but officials are unable to get at them.

"We need unfettered access to the site," the UN official told reporters. "The roads to the site are not reachable currently."

Danish and Norwegian ships are to take the chemicals to a US ship for destruction at sea, along with sites in Finland, the US and Britain.

Once those containers are loaded "the operation can be concluded very quickly," Kaag said, adding that "the clock is ticking," and that the deadline may have to be pushed back.

Huge swathes of Syria remain embroiled in brutal conflict as the government battles rebels seeking to overthrow President Bashar al-Assad.

Syria agreed to hand over its chemical weapons for destruction after Washington threatened military action in response to a chemical weapons attack outside Damascus in August 2013.

The United States and the Syrian opposition blamed the attack, which reportedly killed hundreds of people, on the Syrian regime.

Damascus has denied involvement, but under pressure agreed to dismantle its chemical weapons program.

Meanwhile, Syria has told the OPCW that it has been unable to complete the handover of its chemicals because of ongoing fighting in the war-wracked country.

But Britain on Thursday accused Damascus of foot-dragging, saying it was deliberately "holding up the next steps of transportation, trans-shipment, unloading and ultimately destruction" of the remaining chemical arms.