Syria's opposition National Coalition said Friday it was "deeply sceptical" about the government's decision to join a chemical weapons ban and urged a tough UN resolution to enforce the measure.

Its statement came a day after Damascus filed documents at the United Nations seeking to join the international convention banning chemical weapons.

The move was part of a Russian-led deal intended to head off US military action against the regime, which has denied widespread charges of having used chemical weapons in an August 21 attack which left hundreds dead.

Syrian rebels on the ground have rejected the deal while the National Coalition, on the political front, has criticised it.

"The Syrian Coalition is deeply sceptical about the Assad regime's signing of the Chemical Weapons Convention on Thursday," the umbrella group said.

"Such a gesture comes as too little, too late to save civilians from the regime's murderous intent and is clearly an attempt to evade international action as well as accountability in front of the Syrian people."

The group said any UN Security Council resolution should "enforce compliance through clearly defined timelines and consequences."

"It is vital the threat of force stays on the table. For a UNSC resolution to be anything other than a get-out-of-jail-free card for the regime, it must be enforceable under Chapter 7," allowing military action, the statement said.

The Coalition warned against allowing Syria to play for time and exploit the deal to continue "its policy of widespread violence against civilians."

"This can only be accomplished by making it clear to the regime it must fulfil obligations within a set time period, and that international military action will be the consequence of non-compliance."