The United States warned on Tuesday the Syrian regime and its allies against the use of chemical weapons in the ongoing war in the country. It vowed a “swift and appropriate” response if regime leader Bashar Assad used the prohibited weapons again. The White House also said in a statement that it was closely monitoring developments in Syria’s opposition-held Idlib region, where the regime is expected to mount an offensive that could spark a humanitarian disaster. President Donald Trump in the past year and a half has twice ordered US-led air strikes against targets in Syria in response to the regime’s use of chemical weapons against civilians. In a tweet on Monday, Trump warned Assad and his allies Iran and Russia not to “recklessly attack” the Idlib province and warned that hundreds of thousands of people could be killed. White House spokeswoman Sarah Sanders said the United States is closely monitoring the situation in Idlib province and that Trump has warned an attack there would be a reckless escalation of an already tragic conflict and would risk the lives of hundreds of thousands of people. She said the United States will work with its allies to find a lasting diplomatic solution to resolve the hostilities in Syria under the auspices of United Nations Security Council Resolution 2254. Russian warplanes resumed on Tuesday strikes against Idlib after a 22-day halt. At least nine civilians were killed in the raids, said the Syrian Observatory for Human Rights. The British-based monitor said that five children from the same family were killed in the strikes, while 10 people were wounded.