"USA counterparts have been saying for the past few days Russian Federation is a guarantor of Syria's chemical disarmament, as if the United States has no bearing on that at all", Lavrov told a news conference following a meeting with Syrian Foreign Minister Walid Muallem and Iranian Foreign Minister Mohammad Javad Zarif.
The deal ensured flight safety over Syria where both Russian and US planes have been carrying airstrikes against the Islamic State (IS) group and other radical groups for years.
It's suspected that sarin gas was used in an attack on a northern Syrian town at the start of April. At least 84 people were killed.
The head of Iranian diplomacy has in recent days talked to many of his counterparts in a bid to persuade them of the need to open an impartial investigation.
"Our impression is that the West, principally the US, is colluding with terrorists, and that it made up this whole story to serve as the pretext for an attack", Assad said in the interview released Thursday.
The central issue of the talks will be discussing measures to prevent the undermining of political settlement efforts in Syria in the context of the USA strike, the Russian foreign ministry said.
In Russia's view, the probe conducted by the Organization for the Prohibition of Chemical Weapons should be widened to include experts from many nations, he said.
US President Donald Trump: "Things will work out fine between the United States of America. and Russian Federation". "Frankly, Putin is backing a person that's truly an evil person", Trump said, referring to Assad.
Following a meeting between U.S. secretary of state Rex Tillerson and Russian counterpart Sergei Lavrov on Wednesday, Donald Trump warned relations were "at an all-time low". Putin and his allies are by far the dominant military power in eastern Syria.
The three ministers also discussed the beefing up of US forces on Jordan's border with Syria.
The missile attack on Assad's air base hurt Russia's prestige.
Tension between Russian Federation and the USA has heightened following the missile strike launched last Friday by the US against the Shayrat Air Base in central Syria, with Moscow announcing suspending the flight safety agreement with Washington.
While the most immediate US-Russian dispute concerns culpability for the chemical weapons, broader disagreements over everything from Ukraine to Russia's support for once-fringe candidates in European elections are among other sore points.
For his part, President Assad said in an interview with the AFP published on Thursday that the chemical attack was "100 percent fabrication".
Separately, we must be clear that chemical weapons, like biological and nuclear weapons, constitute a class of weapons altogether different from conventional warfare, and their use is wholly unacceptable at any time and at any scale.