Update | Syria arms inspectors face unprecedented danger: UN

The United Nations has warned that inspectors face a year-long mission of unprecedented danger in trying to destroy Syria's chemical weapons arsenal, as rebels launched a major offensive in the northwest.

In a report to the UN Security Council, Secretary General Ban Ki-moon said it would take 100 foreign experts to complete "an operation the likes of which, quite simply, has never been tried before". Ban set out a blueprint for the most dangerous disarmament operation ever, even as the United States and Russia praised steps taken so far by President Bashar al-Assad's government.

But a major rebel assault in northwest Syria highlighted the looming threats to inspectors from the joint UN and Organisation for the Prohibition of Chemical Weapons mission.

The two international bodies have had to speedily gather a team after a Security Council resolution endorsed a Russian-US disarmament plan on September 27. The plan was launched after a chemical weapons attack near Damascus on August 21, in which hundreds died.

US President Barack Obama initially threatened a military strike, but when he struggled to win international or even domestic support, agreed to a Russian disarmament plan.

Russia and the US have the ambitious aim of seizing and destroying Syria's huge chemical weapons arsenal, estimated at 1,000 tonnes, by the middle of next year. The mission will have bases in Damascus and Cyprus.