Syria and Russian Federation have suggested a conventional airstrike by the Syrian army may have hit a depot storing chemical weapons possessed by extremists.
"Within the framework of the Organization for Prohibition of Chemical Weapons and the United Nations, we will insist on the immediate dispatch of inspectors both to the site of the incident and the airbase where our western colleagues claim missiles were loaded with chemical substances", Lavrov said.
The April 4 sarin gas attack in the town of Khan Shaykhun killed scores of people and prompted the U.S. to launch a cruise missile strike on a Syrian air base.
The Syrian government's suspected chemical weapons attack on civilians last week amounts to a "war crime", the US State Department has said.
The US said its strike was in response to what it believed was a chemical attack by the Syrian government against its own people in Idlib.
"If they are serious in fighting terrorists, we're going to be partners, and I said not only the United States", Assad said.
Denouncing a "very barbaric" attack, Trump ordered a strike that saw 59 Tomahawk cruise missiles slam into the airbase in central Syria from where Washington accused Assad's forces of launching the attack.
Russian Federation has staunchly backed Assad's government throughout a civil war that has dragged into a seventh year.
"Frankly, it's a tactic we've seen on Russia's part as well in the past", Toner told a daily press briefing. Citing the importance of U.S. -Russia talks in Moscow on April 13, the foreign minister stressed that "this is an issue to the concern of all parties".
Syria's government invited the Organization for Prohibition of Chemical Weapons to visit the site of the April 4 incident and the airbase that the USA later bombed, Foreign Minister Sergei Lavrov said.
"There's growing evidence that this was staged", Lavrov said at a Moscow news conference with his Iranian and Syrian counterparts on Friday.
"In recent days, the progress in all directions has been threatened as a result of the incident in Idlib followed by an U.S. illegitimate move - a strike on military airfield of Syrian armed forces", Lavrov added, RIA Novosti reported.
Following Donald Trump's "animal" remark for Syrian President Bashar Assad in the wake of the Idlib chemical attack, Syria's senior diplomat has come down heavily on the US President.
Sakat said that according to his contacts inside Syria, Assad has not manufactured new weapons since 2014, but "they don't need any more, they have all they need already".
Lavrov was meeting with his counterparts from Syria and Iran in a show of support for Damascus after US Secretary of State Rex Tillerson visited Moscow earlier this week.