The US says it has seen signs the Syrian government may again be using chemical weapons in its attritional offensive to recapture the last rebel-held pocket of the country, warning the use of any such weapons would incur retaliatory measures from Washington and its allies.

The alleged attack occurred on Sunday on the border of Idlib province, where in recent weeks the Syrian president, Bashar al-Assad, has stepped up an air and ground campaign that has killed at least 150 people and displaced more than 200,000 from their homes.

Local reports said four hardline Hayat Tahrir al-Sham (HTS) fighters were injured by chlorine released by munitions dropped near the village of Kabana on Sunday.

The use of chlorine was alleged by the Idlib province health directorate but has not been corroborated by monitoring groups or international media. No casualties were reported in the mountainous area, where most residents have fled because of fighting.

Morgan Ortagus, a state department spokesman, said in a statement on Tuesday: “We are still gathering information on [Sunday’s] 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.”

Ortagus said the alleged attack was part of a violent campaign by Assad that violated a ceasefire that has protected millions of civilians in the greater Idlib area since September last year.

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“The regime’s attacks against the communities of north-west Syria must end,” the statement said. “The United States reiterates its warning, first issued by President Trump in September 2018, that an attack against the Idlib de-escalation zone would be a reckless escalation that threatens to destabilise the region.”

France’s foreign ministry said it had noted the allegations “with a degree of alarm” and said they should be investigated. “We have full confidence in the Organisation for the Prohibition of Chemical Weapons,” it added.

The Trump administration has twice bombed Syria over Assad’s alleged use of chemical weapons, in April 2017 and April 2018. In September, a senior US official said there was evidence chemical weapons were being prepared by Syrian government forces ahead of an expected Idlib offensive.

A US official, speaking on the condition of anonymity, said the Syrian government had a history of resorting to chemical weapons when fighting intensified. The official, however, was not aware of any confirmation of what substance was allegedly used, if at all, in Sunday’s incident in Kabana, and said the US government was still gathering information.

There was no immediate comment from the Syrian government on the US statement.

In March, Syrian state media cited a hospital in government-held Hama as saying 21 people suffered choking symptoms from poison gas after rebels shelled a village.

The state department statement also accused the Syrian government and its allies in Russia of “a continuing disinformation campaign … to create the false narrative that others are to blame for chemical weapons attacks”.

“The facts, however, are clear,” the statement said. “The Assad regime itself has conducted almost all verified chemical weapons attacks that have taken place in Syria – a conclusion the United Nations has reached over and over again.”