Russia has "indisputable" data that the alleged chemical attack in the Syrian town of Douma was staged by "foreign agents", Foreign Minister Sergey Lavrov said on Friday.
"We expect the mission to go without any delay to Douma where Russian specialists who examined this place have not found any confirmations of the use of chemical weapons," he said.
"We have the irrefutable data that this was staged... and the special services of a country, which is now seeking to be in the first ranks of the Russophobic campaign, were involved in this staged event," he said.
The US and France said they had proof it took place and, alongside the UK, were considering military retaliation.
Russia, which has military forces deployed in Syria in support of the government, warned that US air strikes risked starting a war.
Russia accused Washington of putting international peace at risk and requested a UN meeting in New York later in the day but it was not confirmed that it will happen. The White House said it was continuing to assess intelligence and talk to its allies about how to respond.
"The immediate priority is to avert the danger of war," said Moscow's UN ambassador Vassily Nebenzia on Thursday.
The delegation from the OPCW will start its investigations in Syria's Eastern Ghouta region on Saturday but few details were expected to be released about its movements for safety reasons.
The suspected attack, denied by Syrian President Bashar al-Assad's government, was carried out last week, reportedly killing over 70 people.
The Violations Documentation Centre, which records alleged violations of international law in Syria, said bodies were found with foam at the mouth, discoloured skin and burns to the eyes.
On Thursday, US officials said they had blood and urine samples from victims which had tested positive for chlorine and a nerve agent, according to a NBC News report.
French President Emmanuel Macron also said he had "proof" that the Syrian government had attacked Douma with chemical weapons.
In the UK, Cabinet ministers said that it was "highly likely" the Assad regime was responsible for the attack. UK Prime Minister Theresa May and US President Donald Trump agreed on the need to deter chemical weapon use in Syria.
Trump had also said last week that Putin bore responsibility for the "atrocity" in Douma.
On Wednesday, the US President said the missiles were "coming", but on Thursday he tweeted that he had "never said when". It "could be very soon or not so soon at all".
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