Originally posted by the Voice of America. Voice of America content is produced by the Voice of America, a United States federal government-sponsored entity, and is in the public domain. US Warns Syria Against Chemical Attack on Idlib by VOA News The U.S. said Tuesday it would "respond swiftly and appropriately" if Syria, with Russian and Iranian support, uses chemical weapons in an attack on Idlib province, the last rebel stronghold in Syria. The White House said in a statement the U.S. is "closely monitoring" the fate of Idlib, where it said "millions of innocent civilians are under threat of an imminent ... attack" by the regime of President Bashar al-Assad. It said President Donald Trump "has warned that such an attack would be a reckless escalation of an already tragic conflict." The statement warned against the use of chemical weapons, but made no mention of how the U.S. would react to a conventional military attack on Idlib. The U.S. said it "will continue to work tirelessly with its allies to find a lasting diplomatic solution to resolve the hostilities in Syria" under a United Nations Security Council resolution. Russia joined Iran in expressing support for Syria's impending operation to retake control of Idlib. Kremlin spokesman Dmitry Peskov said Idlib is a pocket of terrorism, and that the situation there is undermining efforts to find a political resolution to the Syrian conflict. That follows statements Monday by Iran's Foreign Ministry saying Idlib should be cleared of "terrorists." Iranian media also quoted Foreign Minister Mohammad Javad Zarif saying during a visit to Syria that Idlib should be put back under the control of the Syrian people, and that reconstruction efforts in Syria and the return of refugees should go forward. There are about 3 million people in Idlib and the United Nations has said it is worried about the potential humanitarian toll that may come with a Syrian military campaign there. Trump warned Syria in a tweet Monday night to not "recklessly attack" Idlib. He also called on two major backers of Assad's military to restrain their actions. David Lesch, a history professor at Trinity University in San Antonio, told VOA that while the Trump administration is "ratcheting up the pressure a little bit" beyond its previous admonitions to not use chemical weapons, U.S. influence on what happens in Idlb is limited. "Frankly speaking, I don't think there's anything the United States can do about it. I think Russia and the Syrian government and their allies are dead set on taking over Idlib, either in a phased way or in an all-out massive invasion," Lesch said. Syria has been at war since early 2011 with a multitude of parties -- including pro-government forces, rebel groups and militants -- all fighting for control over various areas. Assad's forces, backed by military support from Russia and Iran, have recaptured major cities in recent years, often involving agreements with both opposition fighters and civilians that allowed them to flee to Idlib. Assad's government has long referred to any opposition fighters as "terrorists." The Idlib area includes both rebel groups and militants such as the Nusrah Front. The presidents of Iran, Russia and Turkey are set to hold a meeting Friday in Tehran to discuss the situation in Idlib. VOA's Victor Beattie contributed to this report.