The British embassy to the Netherlands on Monday said Russia and Syria had not yet allowed members of a fact-finding mission to enter the site of a chemical attack in Syria.
A team from the Organisation for the Prohibition of Chemical Weapons (OPCW) was expected to visit Douma, in Syria's Eastern Ghouta, where a chemical attack - allegedly using chlorine and sarin - killed at least 40 people and wounded hundreds.
President Vladimir Putin's spokesman Dmitry Peskov said the claims were "groundless" and that Moscow was in favour of "an impartial investigation" into the attack.
Russia's Deputy Foreign Minister Sergei Ryabkov's told reporters that the only real obstacle faced by the OPCW team in Syria was the "consequences of the illegal, unlawful military action." A reference to the punitive airstrikes carried out by the US, UK and France on Saturday morning. He also said the team could not access the site without an appropriate UN permit.
Meanwhile Syrian state TV aired interviews with doctors claiming they had found no trace of poisonous gas in Douma.
The watchdog was scheduled to to convene on Monday to discuss their team's findings, as its inspectors probed the site in the Damascus suburbs.
Moscow has vowed not to interfere in the team's work and hit out at the United States, saying the weekend strikes on three facilities in Syria were a bid "to undermine the credibility" of the mission.
Mr Peskov also dismissed French President Emmanuel Macron's claims that the weekend air strikes had driven a wedge between Ankara and Moscow.
As the on-the-ground investigation fails to get under way, the fallout from the US-led response continues to reverberate, with French President Emmanuel Macron claiming to have persuaded President Donald Trump to keep his troops in Syria.
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In London, British Prime Minister Theresa May is due to face an emergency parliamentary debate on Monday over her country's part in the operation.
The US-led strikes were the biggest international attack on President Bashar Al Assad's regime since the start of Syria's seven-year war.
They have risked confrontation with Moscow, the Syrian regime's top ally, with President Putin warning that fresh attacks would spark "chaos", while Washington vowed economic sanctions against Russia rather than further military action.
US, French and British missiles destroyed sites suspected of hosting chemical weapons development and storage facilities, in a move lauded by President Trump as "perfectly executed" - although the buildings were mostly empty and both Damascus and Syria's opposition rubbished its impact.
The Western trio swiftly reverted to diplomatic efforts, with leaders facing flak at home over the punitive attack.
But their unified stance appeared to be shaken on Sunday when Washington knocked back President Macron's claim that Paris had convinced Mr Trump to stay engaged in Syria "for the long-term".
White House spokeswoman Sarah Sanders said the US mission "has not changed" and Mr Trump wanted troops home "as quickly as possible".
Saturday's strikes came just hours before the OPCW team arrived in Damascus.
The inspectors will face a difficult task, with all key players having pre-empted their findings, including Western powers, which justified the strikes by claiming they already had proof such weapons were used.
The team will also have to deal with the risk that evidence may have been removed from the site, which lies in an area that has been controlled by Russian military police and Syrian forces over the past week.
"That possibility always has to be taken into account, and investigators will look for evidence that shows whether the incident site has been tampered with," Ralf Trapp, a consultant and member of a previous OPCW mission to Syria said.
The OPCW declared that the Syrian government's chemical weapons stockpile had been removed in 2014, only to confirm later that sarin was used in a 2017 attack in the northern town of Khan Sheikhun.
