A team with the Organization for the Prohibition of Chemical Weapons arrived in Damascus on Saturday at the invitation of the government to investigate the alleged chemical attack, which prompted US -led strikes against Syrian chemical weapons facilities over the weekend.
U.S. Marine Lt. Gen. Kenneth F. McKenzie, director of the Joint Staff at the Pentagon, said the allied airstrikes "took out the heart" of Assad's chemical weapons arsenal.
The claim by Russian Federation comes at a time when the British Prime Minister, Theresa May, is likely to face heavy political pressure over her decision to bomb Syria's chemical weapons program.
The suspicions of tampering heightened concerns that the truth about the April 7 attack may never be known.
Witnesses and Western governments say helicopters dropped chemical bombs that killed many children and women hiding in cellars from bombardment.
Sergei Lavrov said: "I can guarantee that Russian Federation has not tampered with the site".
In interviews broadcast by the pro-government Al-Ikhbaria television channel, 13 medical workers, including nine doctors, said the symptoms they treated suggested that the patients were suffering from asthma, not the effects of a poison gas.
Earlier, Britain's delegation to the OPCW accused Russian Federation and the Syrian government of preventing the global watchdog's inspectors from reaching Douma.
The military action is not about intervening in Syria's civil war or changing its government, she said.
The inspectors aim to collect samples, interview witnesses and document evidence to determine whether banned toxic munitions were used, although they are not permitted to assign blame for the attack. "And they will go directly to any sort of companies that were dealing with equipment related to Assad and chemical weapons use".
"Unfettered access (is) essential", the statement said.
The Russian delegation to the OPCW have said that these allegations were groundless and reinforced that Russia has always supported an investigation. "Russia and Syria must co-operate".
Iran and Russian Federation should not allow the "fire of a new tension" to flare up in the region, Rouhani said, adding that the airstrikes were an "invasion" aimed at "emboldening defeated terrorists", IRNA reported.
Ryabkov said that Russia's lower house of parliament is considering legislation that would give the Kremlin the power to restrict United States imports, RIA reported.
United States' Ambassador to the United Nations Nikki Haley said Sunday the Trump administration will announce more sanctions against Russian Federation as soon as Monday.
The US, Britain and France launched a series of air strikes against Syrian government targets in the early hours of Saturday in retaliation for the alleged gas attack.
The other two were suspected chemical weapons facilities at Him Shinshar near Homs.
The ministry additionally pointed to previous alleged use of chemicals by the rebels in fighting with Syrian government troops.
Members of the 41-seat executive council of the OPCW were due to discuss the alleged use of prohibited toxins in Syria, but were not expected to reach any agreement about a response.