SEOUL (AFP) - UN chief Ban Ki Moon said on Monday there was no time to waste as UN experts prepared to investigate a suspected Syrian chemical attack, with Washington suggesting the probe was already too late.

"Every hour counts. We cannot afford any more delays," Mr Ban told reporters in Seoul, hours before a UN team was due to inspect the site of last week's alleged chemical attack near Damascus.

The Syrian authorities green-lighted the inspection on Sunday, but US officials said it was too little, too late, arguing that persistent shelling of the site in recent days had "corrupted" the site.

"The world is watching Syria," the secretary general said, stressing once again that the UN experts must be allowed to conduct a "full, thorough and unimpeded" investigation.

"We cannot allow impunity in what appears to be a grave crime against humanity," said Mr Ban who was wrapping up a five-day visit to his homeland, South Korea.

Syria's opposition says more than 1,300 people died when regime forces unleashed chemical weapons against rebel-held towns east and southwest of Damascus Wednesday, while Doctors Without Borders said 355 people had died of "neurotoxic" symptoms.

Damascus has strongly denied it carried out such an attack, instead blaming the rebels.

Mr Ban said the success of the UN investigation was "in everyone's interest" and would have an important "deterrent impact" on the possible future use of chemical weapons.

"If proven, any use of chemical weapons by anyone under any circumstances is a serious violation of international law and an outrageous crime," he added.