The U.S. State Department warned the Bashar al-Assad government in Syria that its military operations were being closely watched, specifically concerning allegations that it used chemical weapons against rebels.

"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," State Department spokeswoman Morgan Ortagus said in a statement Tuesday. "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."

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Ortagus said the alleged attack was part of a violent campaign waged by President Assad and violates a ceasefire that has protected millions in the Idlib area.

"The regime's attacks against the communities of northwest Syria must end," she said, adding that this is the Assad government's second warning following the one made by President Donald Trump in September 2018.

She said Russia and Assad have previously fabricated allegations of chemical attacks by the rebels in order for Assad to justify its "own barbaric chemical weapons attacks."

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