(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.
"The terrorists operating in the Idlib de-escalation zone have a significant amount of toxic substances, which the militants equip ammunition for the performances of so called use of chemical weapons against the civilian population by the government troops." the Russian statement said.
Ortagus pushed back at the allegations: "Russia's recent allegations against the White Helmets and others are part of a continuing disinformation campaign by the Assad regime and Russia to create the false narrative that others are to blame for chemical weapons attacks that the Assad regime itself is conducting."
"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. The facts, however, are clear: the Assad regime itself has conducted almost all verified chemical weapons attacks -- a conclusion the United Nations has reached over and over again," she added.
Thousands of anti-government forces remain in Idlib, Syria, where regime troops backed by Russian airstrikes have ramped up attacks in recent days.
US officials have acknowledged that many of the remaining rebel groups in the area have links to extremist groups such as the local al Qaeda affiliate, but officials are also wary of the humanitarian fallout that will result from large-scale regime attacks, given the large presence of civilians.