Russia will argue at the United Nations that an apparent chemical attack that left scores dead in Syria was in fact contamination caused by rebels' chemical munitions, Kremlin spokesman Dmitry Peskov said on Wednesday.

Russia has already suggested it would publicly stand by Syrian President Bashar al-Assad.

"Russia and its armed forces will continue their operations to support the anti-terrorist operations of Syria's armed forces to free the country," Peskov told reporters.

"In our work with the Security Council, Russia will, as part of its argument, present the facts which have already been laid out by our defence ministry," he added.

A Russian defence ministry statement said earlier that poison gas which killed scores of people in northwestern Syria had leaked from an insurgent chemical-weapons depot after Syrian warplanes hit it.

Washington, Paris and London have drawn up a draft U.N. Security Council statement condemning the attack and demanding an investigation. Russia has the power to veto it, as it has done to block all previous resolutions that would harm Assad.