US Defence Secretary Jim Mattis blamed the Syrian government for delays in inspectors reaching the sites and said it has a history of trying to "clean up the evidence before the investigation team gets in."
"We are very much aware of the delay that the regime imposed on that delegation but we are also very much aware of how they have operated in the past and seal what they have done using chemical weapons," Mattis said before the start of a meeting with his counterpart from Qatar.
The United States, Britain and France fired missiles at Syrian targets last weekend in retaliation for the suspected chemical use. They say the arrival of the inspectors is being held up by Syrian authorities who now control the area, and that evidence of the chemical attack may be being destroyed.
Damascus and its ally Moscow deny that any gas attack took place, that they are holding up the inspections or that they have tampered with evidence at the site.
Britain's ambassador to the OPCW Peter Wilson said it was now unclear when the inspectors would be able to reach it.
The rebel group based in Douma announced its surrender hours after the suspected chemical attack, and the last rebels left a week later, hours after the Western retaliation strikes.
-Reuters, AAP
