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.

Ban highlighted the threat to the experts and Syrian civilians from the sarin, mustard gas and other chemical horrors that are to be moved amid the "carnage" of Assad's daily battle with rebels.

The experts will have to work in "dangerous and volatile" conditions, particularly in urban areas such as Damascus, Homs and Aleppo.

"Heavy artillery, air strikes, mortar barrages and the indiscriminate shelling of civilian areas are commonplace and battle lines shift quickly," Ban said.

Syria has already made a declaration of its weapons facilities. And the UN resolution set a November 1 deadline for the eradication of production and chemical mixing facilities. That work started on Sunday when the first "missile warheads, aerial bombs and mixing and filling equipment" were cut up and destroyed, according to the UN.

Western countries blame Assad for the August 21 chemical attack, while his ally Russia blames the rebels.

US Secretary of State John Kerry meanwhile welcomed progress made so far in the disarmament operation.

"It's extremely significant that yesterday, Sunday, within a week of the resolution being passed, some chemical weapons were being destroyed," he said in talks with Russia's Foreign Minister Sergey Lavrov in Bali.

"I think it's a credit to the Assad regime, frankly. It's a good beginning and we welcome a good beginning."

On the ground on Monday, Assad's army reopened the only supply route linking central Syria with the northern city of Aleppo after a year of fierce fighting.

But rebels launched a major assault on two key military bases in northwestern Syria, with at least 14 people killed in the heaviest fighting in the area for months, activists said.

Rebel rocket and mortar fire killed at least 10 soldiers and destroyed three tanks, said the British-based Observatory. Four rebel fighters were killed.

Assad forces used helicopters to bombard rebel positions around the two bases with barrels filled with explosives.