(CNN) The US State Department issued a warning to the government of Bashar al-Assad in Syria on Tuesday, saying the US is closely watching the regime's military operations against a rebel enclave in northwest Syria and is looking into allegations that Assad's troops have used chemical weapons in recent days.

"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 spokesperson 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 added.

The Trump administration ordered retaliatory strikes against Syrian regime targets in 2017 and 2018 after reports of chemical weapons attacks by Assad's forces.

Earlier on Tuesday the Russia Ministry of Defense accused Syrian rebels, including "specialists who have been trained in Raqqa city under the guidance of US intelligence officers," of planning a "false flag" chemical weapons attack to frame the Syrian regime in Idlib.

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