SEOUL (AFP) - The use of chemical weapons in Syria would constitute a "crime against humanity," UN Secretary General Ban Ki Moon said on Friday, warning of "serious consequences" if their use was proven.

"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law," Mr Ban said in impromptu remarks at a scheduled event in the South Korean capital Seoul.

"Such a crime against humanity should result in serious consequences for the perpetrator," he added.

A UN inspection team is already on the ground in Syria and Mr Ban urged the Syrian authorities to cooperate with an immediate investigation into an alleged chemical weapons attack near Damascus which the opposition says killed hundreds.

"This is a grave challenge to the entire international community - and to our common humanity, especially considering it occurred when the United Nations expert mission is in the country," Mr Ban said.

"I can think of no good reason why any party - either government or opposition forces - would decline this opportunity to get to the truth of the matter."

Footage distributed by activists showing unconscious children, people foaming around the mouth and doctors apparently giving them oxygen to help them breathe has triggered revulsion around the world.

Mr Ban said the report of the attack was "very alarming and shocking" and underlined the need for all sides in the Syrian conflict to lay down their arms and begin a dialogue.