Amid a fierce war of words between Russia and the US and its allies, Moscow has said that a team of chemical experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) will inspect the site of a suspected poison gas attack in Syria on Wednesday.
The announcement came as Russia rejected US accusations of tampering with evidence in an investigation over the alleged use of chemical weapons by the Syrian government in Douma, near Damascus, on April 7.
"Tomorrow [Tuesday] the security services of the United Nations … will test the routes. And on Wednesday is when we plan the arrival of the OPCW experts", a senior Russian official said in The Hague on Monday.
Earlier, inspectors from the OPCW claimed to have been prevented by Syrian forces and their Russian backers from accessing the site earlier on Monday.
"Russia and Syria have not yet allowed access to Douma," the British delegation said in a tweet on Sunday before adding that “unfettered access [is] essential."
A statement by the director-general of the OPCW on Monday said that Syrian and Russian officials who "participated in the preparatory meetings in Damascus have informed the FFM [Fact-Finding Mission] that there were still pending security issues to be worked out before any deployment could take place."
Against this backdrop, Heather Nauert, US state department spokesperson, demanded via Twitter that the Russians cease the "disinformation" and give the inspectors access to the site.
"Chemical weapons were used on Syrian men, women, and children in Douma. Reports that OPCW weapons inspectors require special UN passes are completely false. Syria and Russia need to stop the disinformation and allow unfettered access to the attack sites," she said.
Kenneth Ward, the US representative to the OPCW, also expressed concern at Russia's visit to the attack site at a meeting at The Hague on Monday.
WATCH: Chemical attack - What it affects the war in Syria
"It is our concern that they [Russians] may have tampered with it with the intent of thwarting the efforts of the OPCW Fact-Finding Mission to conduct an effective investigation," Ward was quoted as saying by Reuters news agency.
However, Sergey Lavrov, Russia's foreign minister, maintained Moscow did not interfere with the investigation.
"I can guarantee that Russia has not tempered with the site," Lavrov told BBC in an interview.
Syrian government forces retook Douma last week, gaining full control over the former rebel-held enclave of Eastern Ghouta, Russian military officials announced at the time.
The announcement came just days after the Douma attack, following which the US, UK and France launched a coordinated missile strike on facilities believed to be used to research, develop and store chemical weapons inside Syria.
The Chemical Weapons Convention outlaws the production or stockpiling of chemical weapons.
Syria is a signatory of the treaty. Egypt, Israel, North Korea and South Sudan are the only nonsignatory nations.
'Further barbaric use'
Peter Wilson, the UK's OPCW envoy, has told the watchdog body that failure to act in Syria will cause "further barbaric use of chemical weapons.
Syria has lived through a seven-year civil war that has killed at least half a million people and created an international refugee crisis.
WATCH: What are Syrians saying about the US-led attacks?
"The Syrian regime has an abhorrent record of using chemical weapons against its own people. Chemical weapons use has become an all too regular weapon of war in the Syrian conflict," Wilson said, going on to cite 390 allegations of chemical weapons attacks since 2014.
Russia has impeded international bodies from investigating these attacks, he said.
Russia told the UN last Friday that its experts found no trace of "toxic substance use" during their investigation in Douma.
Vassily Nebenzia, Russia's UN ambassador, said Russia has "clear evidence" that the alleged chemical attack was staged.
For its part, France has said it has evidence Russia was responsible.
Nikki Haley, US ambassador to the UN, has said her country will announce new sanctions on the Russian government in response to its support for Syria's President Bashar al-Assad.