WASHINGTON — The United States said on Tuesday it suspected that Syrian government forces have carried out a fresh chemical attack and it threatened reprisals.

The State Department said it was assessing indications that the regime used chemical weapons on Sunday during its offensive in Idlib, the most significant remaining holdout in Syria of jihadist rebels.

"We are still gathering information on this incident, but we repeat our warning that if the Assad regime uses chemical weapons, the United States and our allies will respond quickly and appropriately," State Department spokeswoman Morgan Ortagus said in a statement.

Russia and Turkey, the key ally of the rebels, in September reached an agreement that nominally protects Idlib amid fears for the safety of some three million people in the northwestern area.

But Syria's former al-Qaeda affiliate, Hayat Tahrir al-Sham, has seized a large part of the province and adjoining areas, triggering a regime offensive that includes strikes by Syrian and Russian airplanes.

Ortagus said that the offensive has "destroyed known health facilities, schools, residences and internally displaced person camps".

"The regime's attacks against the communities of northwest Syria must end," she said.

"The United States reiterates its warning, first issued by President (Donald) Trump in September 2018, that an attack against the Idlib de-escalation zone would be a reckless escalation that threatens to destabilize the region," she said.

Some 180 civilians have been killed in the flare-up since April 30, according to the Syrian Observatory for Human Rights, a British-based monitor.

The United Nations says that tens of thousands have fled their homes.

International inspectors say that Assad's forces have carried out a series of chemical attacks in the course of the brutal civil war, which has killed more than 370,000 people and displaced millions since 2011. — AFP