The use of chemical weapons in Syria would constitute a "crime against humanity", UN chief Ban Ki-moon said on Friday, adding there was "no time to lose" in probing alleged attacks which the opposition says killed hundreds.

Ban described reports of the incidents near Damascus on Wednesday as "very alarming and shocking" and urged the Syrian government to allow a United Nations inspection team, already on the ground in Syria, to begin an investigation without delay.

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

Ban's comments, at a UN event in Seoul, put added pressure on Syrian President Bashar al-Assad after France denounced the "likely" use of chemical weapons.

"Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law," Ban said. "Such a crime against humanity should result in serious consequences for the perpetrator.

"There is no time to waste," Ban said, adding he had instructed his envoy for disarmament affairs, Angela Kane, to travel to Damascus.

"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."

The United States said it has yet to "conclusively determine" chemical weapons were used. US President Barack Obama has ordered his spy agencies to urgently probe the claims, aides said.

Damascus denied it unleashed chemical weapons, particularly at a time when the UN was in Syria to inspect three sites where other such attacks allegedly took place.

It would be "political suicide" to go ahead with such an attack, a senior security source said.

Moscow has said rebels may have released gas to discredit Assad and urged him to agree to a UN inspection. On Wednesday, Russian objections to Western pressure on Syria saw the Security Council merely call in vague terms for "clarity" - a position increasingly frustrated Syrian rebels described as "shameful".

Beijing said on Thursday that UN chemical weapons experts should be objective and "fully consult" with the Syrian government in their work.

The opposition National Coalition says more than 1,300 people were killed in gas attacks.

Syrian activist Abu Ahmad, said over the Internet from Moadamiyet al-Sham, a rebel-held town southwest of Damascus where the deadliest attack allegedly took place, said he helped bury dozens of civilians whose bodies were "pale blue" and who died of "suffocation".

Videos posted online by activists have provoked shock and condemnation around the globe. None of the videos could be verified.

The US State Department said Obama had instructed intelligence services to gather information about the claims.

"Right now, we are unable to conclusively determine CW use," said State Department spokesperson Jen Psaki.

AFP-Reuters