The UK says inspectors from the Organization for the Prohibition of Chemical Weapons were blocked Monday from entering the site in Douma, despite having UN clearance.
The U.S. says that is because Syria and its allies are trying to tamper with evidence on the ground.
In an interview with the BBC, Russian Foreign Minister Sergei Lavrov denied that was the case.
He blamed the delays on retaliatory U.S.-led missile strikes on Syria over the weekend.
He also said the U.S. and its allies had no proof of the alleged poison gas attack, saying that any evidence they claim to have is from media reports and social media.
The U.S. and France say they have proof such an attack took place, but haven't yet made that evidence public.
Inspectors from the OPCW would be able to establish what happened and who was to blame.
They traveled to Syria last week to inspect the site, but have yet to gain access to Douma, which is now under government control.
But the Russian military said they will be allowed to visit the site on Wednesday.
Meanwhile, President Trump has reportedly put a stop to plans to impose additional sanctions on Russia over its role in the alleged attack.
U.S. Ambassador to the United Nations Nikki Haley had told CBS interview that sanctions would be announced on Monday.
U.S. media have quoted White House officials as saying that Trump conferred with his national security advisers after Haley's announcement, but was uncomfortable with approving economic sanctions at this time.
Ro Aram, Arirang News.