The Organisation for the Prohibition of Chemical Weapons will be given access to an alleged chemical attack site in Syria on Wednesday, Russia says.
Experts arrived in Damascus on Saturday, but they have been unable to visit the nearby town of Douma because of “security issues” cited by Russia.
US officials have raised concerns that Russia, the Syrian government’s ally, might have tampered with the site.
The government insists the incident in Douma on 7 April was fabricated.
Opposition activists, medical organisations and rescue workers say more than 40 people were killed when aircraft dropped barrel bombs filled with toxic chemicals on the town while it was still held by rebel forces.
The US, UK and France say that, based on open source information and their own intelligence, they are confident chlorine and possibly a nerve agent were used.