UNITED NATIONS - UN Secretary-General Ban Ki-moon on Tuesday sent a letter to Syrian President Bashar al-Assad to convey his concerns over chemical weapons, a UN spokesman told reporters on Wednesday.

"The secretary-general has grave concerns about the existence and possible use of chemical weapons," said Martin Nesirky, Ban's spokesperson, at the daily briefing.

"The secretary-general has conveyed those concerns in writing already some months ago to President Assad and that he had done so again in a letter handed over to the Syrian authorities yesterday, " he said.

The spokesperson also reiterated the UN chief's view that the fundamental responsibility of the Syrian government is to safeguard any such stockpiles, and that the use of such weapons would be "an outrageous crime with dire consequences".

After Syria acknowledged in July that it had chemical and biological weapons, the UN chief expressed his concern over the possibility of Syria's resort to chemical weapons, calling such an action "reprehensible."

On Nov 27, Ban further called on Syria and other seven countries to join the Chemical Weapons Convention "without delay."

The Syrian government has said it would not use the weapons against its "own people".