International chemical weapons inspectors have yet to be allowed into the Syrian town of Douma, the scene of a suspected chemical weapons attack that left 75 people dead and drove the U.S., France, and the U.K. to carry out missile strikes on Syria Friday night.

Russia and Syria have blocked access to investigators who arrived in Damascus over the weekend, Ahmet Üzümcü, director general of the Organisation for the Prohibition of Chemical Weapons (OPCW), told the executive council at a meeting on Monday.

Now nine days after the alleged attack and a week since Russia deployed its own experts, the U.S. fears the Russians have tampered with the site.

"We are concerned that they may have tampered with it, with the intention of thwarting the efforts of the OPCW fact-finding mission to conduct an effective investigation," Kenneth Ward, the U.S. ambassador to OPCW, said at the meeting. "This raises serious questions about the ability of the fact-finding mission to do its job."

Russian Deputy Foreign Minister Sergei Ryabkov quickly denied Monday that Russia was holding up investigators, instead arguing the global watchdog inspectors were not being allowed into Douma because they didn’t have a U.N. permit, according to the Washington Post.

“It is the lack of approval by the U.N. Department for Safety and Security for OPCW experts to visit the site in Douma that is the problem,” Ryabkov said. “As far as I understand, what is hampering a speedy resolution of this problem is the consequences of the illegal, unlawful military action that Great Britain and other countries conducted on Saturday.”

Russia has repeatedly denied the April 7 chemical attack by the Assad regime, instead placing the blame on the West and going so far as to say it was a staged provocation.

Russia told the U.N. Security Council on Friday its experts found no evidence "toxic substance use" in the area, and instead it had proof that the U.K. had staged the attack.

"We have evidence that proves Britain was directly involved in organizing this provocation," Igor Konashenkov, Russia's defense ministry spokesman, said.

The OPCW has recorded more than 390 allegations of chemical weapons use in Syria since its fact-finding mission was created in 2014, the U.K.’s envoy to OPCW told the council on Monday.

“Since 2016, Russia has sought to undermine every OPCW investigation into allegations of Regime chemical weapons use,” Peter Wilson, the U.K. ambassador to OPCW. said in a statement. “Yet again, Russia is spreading conspiracy theories and misinformation designed to undermine the integrity of the OPCW’s fact-finding mission.”

American, French, and British planes and warships launched missiles on Friday aimed at three targets in Syria related to the development and storage of chemical weapons.

President Trump celebrated on Twitter less than 24 hours after his announcement, calling the strikes “perfectly executed.”