"We do have numerous sources including interviews with those present during the attack that did report that a number of opposition fighters were taken to local hospitals and presented symptoms that were consistent with chemical exposure", Ortagus said. "We know, of course, that this is a pattern of behavior unfortunately by the Assad regime, but we don't have any definitive conclusions yet as we continue to investigate".
READ MORE: Terror Chemical Attack Plotted in Syria's Idlib, Aleppo — Russian MoD
On Tuesday, the US State Department said that Washington was gathering information regarding what it suspects was a chlorine attack in northwest Syria on 19 May. It added that the United States saw signs that the Syrian government might be using chemical weapons.
On Friday, the Russian Defence Ministry said militants of the Nusra Front terror group were preparing to stage a false-flag provocation in Syria's Idlib province to frame the Russian Aerospace Forces and the Syrian army for allegedly attacking civilians with chemical weapons.
The head of the Russian centre for Syrian reconciliation, Maj. Gen. Viktor Kupchishin, said Tuesday that terrorists operating in the Idlib de-escalation zone possess significant amounts of poisonous substances which are used to fill munitions for use in staged provocations aimed at accusing the Syrian army of "chemical attacks" against civilians.
READ MORE: Russian Air Defences Destroy Rockets Fired at Syria Base From Idlib — MoD
*al-Nusra Front (the Nusra Front, Jabhat al-Nusra) — terrorist organisation banned in Russia and many other countries.