Syrian President Bashar al-Assad's regime has accused armed groups of carrying out a "toxic gas" attack Saturday that left dozens of people struggling to breathe and prompted government ally Russia to launch retaliatory air strikes against "terrorist groups."

Damascus has formally asked for the Organisation for the Prohibition of Chemical Weapons (OPCW) to investigate the alleged attack.

The Pentagon said Assad may try to interfere with the site of the incident and build a narrative to justify attacking the rebel-held stronghold of Idlib, which is currently protected under a 10-week-old truce deal in northern Syria.

"It is essential to ensure that the Syrian regime does not seize on false pretexts to undermine this ceasefire and launch an offensive in Idlib," Pentagon spokesman Commander Sean Robertson said in a statement.

"We caution Russia against tampering with another suspected chemical weapons attack site and urge Russia to secure the safety of the OPCW inspectors so these allegations can be investigated in a fair and transparent manner."

Both the Syrian regime and Russia have blamed "terrorist groups" -- a term Damascus uses to mean both rebels and jihadists -- for Saturday's attack.

Though it is the regime that has been blamed for most deadly chemical weapon attacks in Syria's seven-year war, official media have recently accused fighters in Idlib of planning a chemical attack.

In April, the US, France and Britain launched joint missile strikes on Syrian targets in response to an alleged chemical weapons attack in the town of Douma that left scores of people dead.

The Syrian regime blocked international inspectors from accessing the site for several days after the alleged chemical attack.

"We urge immediate inspection of the alleged site by international investigators, with freedom to interview all involved and unhindered ability to collect evidence," Robertson said.