The U.S. State Department on Tuesday issued a warning to the Bashar al-Assad regime on suspicion it could be conducting a new chemical weapons attack in northwest Syria.

Axar.az reports citing Anadolu News Agency.

"Unfortunately, we continue to see signs that the Assad regime may be renewing its use of chemical weapons, including an alleged chlorine attack in northwest Syria on the morning of May 19, 2019," department spokeswoman Morgan Ortagus said in a statement.

"The United States continues to closely watch the military operations by the Assad regime in northwest Syria, including indications of any new use of chemical weapons by the regime," she added.

Ortagus' statement comes as the regime mounts a new offensive in northwestern Syria the U.S. warned would be a "reckless escalation" that could destabilize the region.

The move threatens to unleash a major humanitarian catastrophe with 180,000 civilians forced to flee in the last three weeks, according to the U.N.

At least 151 civilians have lost their lives in Idlib province since April 25, and more than 405 have been injured.

Some 1.5 million people currently reside in Idlib. Half of them are displaced from other parts of the war-torn country.

Turkey and Russia agreed last September to turn Idlib into a de-escalation zone where acts of aggression would be expressly prohibited.

The regime, however, has consistently broken the terms of a cease-fire there, launching frequent attacks inside the zone.

The U.N.'s emergency relief coordinator, Mark Lowcock, said a full military offensive by the regime could result in the worst humanitarian catastrophe of the 21st century.

Syria has only just begun to emerge from a devastating conflict that began in early 2011 when the regime cracked down on demonstrators with unexpected severity.

"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 department also pushed back against Russian allegations against the civil defense agency, White Helmets, that blames Moscow for helping run a disinformation campaign to blame others for the chemical weapons attacks in Syria.

"At times, Russia and the Assad regime have made these false allegations as a pretext in advance of the Assad regime’s own barbaric chemical weapons attacks," Ortagus said.