The international team has been in the country since Saturday, but has not been allowed to visit Douma, the BBC reported.
The attack on April 7, prompted military strikes on Syrian government targets by the US, Britain and France a week later.
Syria and its ally Russia deny any chemical attack took place - with Russia calling it a "staged thing".
Early on Tuesday, Syrian state media said the country's air defences had responded to a missile attack over the western city of Homs. The missiles targeted Shayrat air base, it said - but did not say who fired the missiles, the BBC report added.
Another report, from the pro-Iranian Hezbollah militia, said that Syrian air defences had intercepted three missiles targeting Dumair military airport, north-east of here.
