The US launched missiles at a Syrian airbase in response to that attack, and President Donald Trump warned after Saturday's strikes that the US was "locked and loaded" to respond if chemical weapons were used again.
Syria agreed to give up its chemical weapons arsenal in 2013 and submit to OPCW inspections.
"Tomorrow (Tuesday) the security services of the United Nations. will test the routes", a senior Russian official said at a press conference in the Russian embassy in The Hague, explaining that the roads were still being cleared of mines.
The airstrikes were conducted hours before inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) were to start their fact-finding mission at the site.
"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", said Ralf Trapp, a consultant and member of a previous OPCW mission to Syria. Russian Federation backs Syrian President Bashar al-Assad's government.
The attack on 7 April prompted military strikes on Syrian government targets by the US, UK, and France a week later.
Missing army man Idrees joins terrorists
The text slapped on the photo of Mir, also known as Chota Abrar, says he joined the home-grown militant outfit on April 15. He had arrived at his native village on April 12 and had been missing since Saturday night, police officials said.
But in an interview for BBC's Hardtalk, Russian Foreign Minister Sergei Lavrov said: "I can guarantee that Russia has not tampered with the site".
The president tells lawmakers that he acted to "promote the stability of the region, to deter the use and proliferation of chemical weapons, and to avert a worsening of the region's current humanitarian catastrophe".
Moscow pointed out that the Western allies ignored evidence provided by Syria and Russian Federation that the alleged chemical attack was actually staged in a "cynical" manner.
Doctors at the hospital where suspected victims were treated told reporters on the tour that none of the patients that night had suffered chemical weapons injuries - they were asphyxiated by dust and smoke in a bombardment. "But it was the world saying that we've had enough of the use of chemical weapons".
The Western leaders faced scrutiny at home over their actions, with Britain's May facing questions over why she did not seek parliamentary approval for the action.
Britain's May will make a statement to parliament on Monday on her decision and will repeat her assertion that Assad's forces were highly likely responsible for the attack. Washington, and its allies - France and the United Kingdom - fired missiles on the Syrian facilities, which they claimed were linked with chemical weapons production. "There is broad-based worldwide support for the action we have taken", May said.