By David Brunnstrom and Idrees Ali

WASHINGTON (Reuters) - The United States sees signs the Syrian government may be using chemical weapons, including an alleged chlorine attack on Sunday in northwest Syria, the State Department said on Tuesday, warning that Washington and its allies would respond "quickly and appropriately" if this were proven.

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

"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," she said.

Ortagus said the alleged attack was part of a violent campaign by Syrian President Bashar al-Assad's forces violating a ceasefire that has protected several million civilians in the greater Idlib area.

"The regime’s attacks against the communities of northwest 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 destabilize the region."

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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 U.S. official said there was evidence showing chemical weapons were being prepared by Syrian government forces in Idlib, the last major rebel stronghold in the country.

"The Assad regime must not repeat the use of chemical weapons in Syria," Commander Sean Robertson, a Pentagon spokesman, said in a statement. "There should be no doubt as to our determination to act strongly and swiftly should the Assad regime use these weapons again in the future," he said.

The State Department statement accused Russia and Assad's forces 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."

A U.S. 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, and said the U.S. government was still gathering information.

There was no immediate comment from the Syrian government on the U.S. 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.

In January, U.S. national security adviser John Bolton warned the Syrian government against using chemical weapons again.

“There is absolutely no change in the U.S. position against the use of chemical weapons by the Syrian regime and absolutely no change in our position that any use of chemical weapons would be met by a very strong response, as we've done twice before," Bolton said at the time.

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(Reporting by Eric Beech, David Brunnstrom and Idrees Ali; editing by Jonathan Oatis)