Russian Foreign Minister Sergey Lavrov on Friday denied allegations that Russia is working with the Syrian government to keep international inspectors away from the chemical weapons attack site in Douma so the evidence can be destroyed.

“As for yesterday’s statement, which said that the US has evidence to prove that Russia and Syria were delaying OPCW experts from gaining access to the town of Douma – if they really have it, let them present it,” Lavrov said at a press conference.

“We have more than once presented the evidence that we have, including pictures and people’s interviews. As for our counterparts, we have only heard allegations about evidence, be it the alleged Douma incident or the Salisbury incident,” Lavrov continued, the latter a reference to the attempted assassination of former spy Sergei Skripal with nerve agents in Britain.

Inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW) attempted to enter Douma to examine the chemical attack sites this week but were kept away by security concerns, including gunfire and an angry mob appearing at one site.

The U.S. State Department on Thursday said it has obtained “credible information” that Russian and Syrian officials are working together to deny inspectors access to Douma, buying time for the attack sites to be “sanitized” and incriminating evidence removed.

British Ambassador to the United Nations Karen Pierce echoed these accusations on Friday, saying there is a “possibility” that Russia and Syria have tampered with evidence in Douma.

“We look to the Russians and the Syrians to uphold the promises they made to the Security Council last week that the inspectors would be allowed in, that they would be escorted, that they would be safe and that they could have free access,” she said.

Pierce did not indicate the British government has any evidence that Russia and Syria are deliberately holding back inspectors or tampering with evidence at the sites. Instead, she stressed the importance of allowing the OPCW team to conduct its work “expeditiously” and file a thorough report.

On Thursday evening, British Prime Minister Theresa May accused the Russians of running a “disinformation campaign” to cover up the chemical weapons abuses in Douma and Salisbury.

“This disinformation campaign is not just aimed at social media and the UK – it is intended to undermine the actual institutions and processes of the rules-based system, such as the Organisation for the Prevention of Chemical Weapons. We must do all we can at every turn to challenge this,” May said.

This week, the director of the largest medical relief agency working on Syria, the Union of Medical care and Relief Organizations (UOSSM) said that doctors who treated patients in Douma were intimidated and threatened by agents of Syrian dictator Bashar Assad, including threats of violence to their families if they tell international inspectors what they have seen.

“There has been a very heavy security presence on the ground ever since the attack and they have been targeting doctors and medics in a very straightforward way. Any medic who tried to leave Douma was searched so vigorously, especially for samples. At one medical point, seven casualties were taken away. The Russian military police were heavily involved. They were directing things,” said one physician.

“We were receiving threats since the siege began, prior to the chemical weapons attack. When the attack took place, things became much more dangerous. They’re wiping out evidence that would prove the crime, and they are forcing doctors and residents who are witnesses to say that nothing took place,” said another medic.

AFP reported on Thursday that photographs circulated by Russia and the Assad regime that purportedly showed “White Helmet” rescue volunteers faking the chemical attack in Douma were themselves fakes. The photos were taken from the set of a propaganda film supported by the Syrian culture ministry that was completed and screened over a month before the chemical attack.