Regime ally Moscow said it would propose a United Nations resolution on a "transparent and honest" investigation with the OPCW's involvement. "Either way, the United States will respond".
State news agency RIA Novosti on Wednesday quoted Andrei Krasov, deputy chairman of the State Duma's defence committee, as saying that Russian Federation will treat a USA airstrike on Syria "not just as an act of aggression but a war crime of the Western coalition".
It ambassador to the UN, Vassily Nebenzia, earlier warned of the potential consequences to Western intervention in Syria, saying that USA attacks on Syria "could lead to grave repercussions".
Nebenzia rejected a U.S. call for the world body to set up a probe to identify the perpetrators of what rescuers called a "poisonous chlorine gas attack" late Saturday in the enclave of Douma, near Damascus.
The United States, backed by Britain and France, has said it is ready to act with or without support from the United Nations, where the Security Council was to vote as early as Tuesday on rival USA and Russian proposals to probe chemical attacks in Syria.
The Syrian government and Russian Federation said there was no evidence that a gas attack had taken place and the claim was bogus.
"They should do something to stop this kind of genocide. I think it is important that we react".
Russian Federation and the U.S. blocked attempts by each other in the United Nations Security Council on Tuesday to set up global investigations into chemical weapons attacks in Syria.
Bolivia voted against it alongside Russian Federation, while China abstained.
Syria has been asked to "make the necessary arrangements for such a deployment", the Organisation for the Prohibition of Chemical Weapons said.
The attack in Douma occurred late on Saturday amid a resumed offensive by Syrian government forces after the collapse of a truce with the Army of Islam rebel group.
The OPCW has said the team of experts will deploy to Syria shortly. This comes after the White House abruptly announced the cancellation of US President Donald Trump's first trip to Latin America, scheduled for later this week, saying he wants to "oversee the American response to Syria".
The incident has sparked worldwide outrage and ratcheted up tensions in the already volatile Mideast, raising the specter of possible imminent American retaliation amid Russia's warnings against any such action, and denials that any chemical weapons attack took place.
The Russian ministry of defence has previously said it will fire at any United States craft that strikes government areas of Damascus, where Russian officials are also based.
Mrs May and Mr Trump had condemned Mr Assad's "vicious disregard for human life" and "agreed not to allow the use of chemical weapons to continue", the White House said.