The use of chemical weapons in Syria would constitute a “crime against humanity” that would reap “serious consequences”, UN Secretary-General Ban Ki-moon has said.

Speaking at an event in Seoul, UN Chief  Ban Ki-Moon said: “Any use of chemical weapons anywhere, by anybody, under any circumstances, would violate international law.

“Such a crime against humanity should result in serious consequences for the perpetrator.”

Mr Ban has decided to send Angela Kane, the UN High Representative for Disarmament Affairs, to Damascus following the alleged chemical attack.

Syria President Bashar al Assad’s regime had agreed last month, following a similar visit, to allow UN experts to investigate three other sites where chemical attacks were alleged to have taken place.

UN spokesman Eduardo del Buey said: “The Secretary-General believes that the incidents reported yesterday need to be investigated without delay.”

A UN inspection team is already on the ground in Syria and Mr Ban urged the regime to co-operate with an immediate investigation into the alleged chemical weapons attack near Damascus, which the opposition has said killed hundreds.

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

Syrian anti-government activists have accused the Assad regime of carrying out the attack using toxic gas, with the number of dead reported to be up to 1,300.

The government has denied it used chemical weapons, calling the allegations “absolutely baseless”.

It comes as Syria’s key ally Russia joined international calls for the inspectors to be given access to the site of the alleged massacre.

Moscow suggested the attack could be a “premeditated provocation” by opposition forces but urged Assad and the UN to agree to a visit to the site in the Damascus suburbs of Ein Tarma and Zamalka.

US Secretary of State John Kerry took part in a meeting with the national security team to discuss the Syria attack.

British Foreign Secretary William Hague has also spoken to Mr Kerry about developments in Syria, the Foreign Office confirmed on Friday.

In June the Obama administration said it had conclusive evidence that Assad’s government had used chemical weapons against opposition forces.

That crossed what Mr Obama called a “red line” and prompted a decision to send arms to Syrian rebels.

State Department spokeswoman Jen Psaki said the intelligence community had been tasked with gathering information on the chemical attack, but that it was “unable to conclusively determine chemical weapons use”.

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