US warns Assad against using chemical weapons as fighting escalates in Syria

As new fighting in Syria's last opposition-held province intensifies, the U.S. is increasingly warning the regime of Bashar al Assad and his backer Russia against escalating the conflict, especially by deploying chemical weapons.

Despite the warnings, the top U.S. envoy expressed some optimism for finding a path forward, in particular with Russian support after Secretary of State Mike Pompeo met with Russian leader Vladimir Putin last week. As the U.S. keeps diplomatic and economic pressure on Assad, Moscow is beginning to come around to an understanding that Assad cannot win the war militarily and will never regain international support, according to U.S. Special Representative for Syria James Jeffrey.

The U.S. is gathering evidence of a possible chlorine gas attack on Sunday and has seen "signs that the Assad regime may be renewing its use of chemical weapons," the State Department said Tuesday.

"The United States and our allies will respond quickly and appropriately" if chemical weapons were confirmed to have been used, spokesperson Morgan Ortagus said in a statement.

President Donald Trump has twice ordered airstrikes on Syrian military targets after the U.S. confirmed the regime had deployed sarin gas on civilians, although previous uses of chlorine did not precipitate an armed response. Ortagus told ABC News chlorine when deployed in warfare is a banned chemical weapon, but she declined to preview any military action.

The State Department also warned against any escalation in Idlib province, the rebel stronghold backed by Turkey that is supposed to be under a ceasefire brokered by it and Russia. In recent weeks, however, Syrian airplanes and artillery, backed by Russian air power, have bombarded sections that the two countries say are infiltrated by terror groups.

The fighting has displaced some 180,000 new Syrians, according to the United Nations, with at least dozens of people killed amid the most intense period of violence in months.

But while the State Department has increased its warnings about an escalation by Assad, President Trump has been quiet. Ahead of what analysts said was a pending offensive by Assad and Russia in September, he issued a tweet warning against the offensive. The White House later promised the U.S. would respond "swiftly and appropriately" if they moved ahead, and days later, Putin and Turkish President Recep Tayyip Erdogan signed a ceasefire.

Trump took credit for that deal, but so far has not weighed in, although Ortagus said the White House was "fully supportive" of the State Department's warnings.

One powerful rebel group struck back Wednesday. Al Nusra Front, which was at one time affiliated with al Qaeda, launched an offensive on a Russian air base with around 500 fighters, seven tanks, and around 30 heavy machine-gun mounted pick-up trucks, according to Reuters news agency.

The attack will likely prompt an even stronger military response from Assad, who U.S. officials say believes he can still win the eight-year old war through force.

That's a view that virtually everyone else has abandoned, according to U.S. Special Representative for Syria James Jeffrey, who said that gives him hope.

"Everybody wants this war to end, everybody wants refugees and [internally displaced persons] to return, and everybody wants the fighting and this danger of an escalation to stop," Jeffrey told the House Foreign Affairs Committee Wednesday, with one exception: Assad himself, who has "shown little willingness to be flexible on any issue and at the moment may be concluding that it's better to sit on a pile of rubble with half his population and 60 percent of his country than compromise."

To that end, Russia has been helpful, according to Jeffrey, who said Moscow knows they need a political solution to rid themselves of the costly conflict and an ally that most of the rest of the world opposes. Pompeo made that clear with a "very strong démarche," per Jeffrey, when they met Putin in Sochi last Tuesday.

"We received assurances from the Russians, some of which they seemed to have been trying to carry out in the days since we were in Sochi. Trying to slow down or stop any military conflict with dozens of groups on the ground is not easy... But we did believe that we made some progress with President Putin," he added.

U.S. sanctions and a promise by America and its allies to withhold any reconstruction funds until Assad allows for a political transition has been instrumental in influencing Russia and key to that future path, Jeffrey said. To that end, he urged Congress to continue to apply economic pressure, including passing the Caesar Syria Civilian Protection Act.

The bill, which was approved by the Senate Foreign Relations Committee Wednesday, would require the president to sanction anyone who does business with the Syrian government or central bank, including those involved in construction, engineering, or energy projects or those providing aircraft or spare aviation parts. It also authorizes the State Department to support the collection and preservation of evidence of war crimes or crimes against humanity to aid in any future investigations and trials.

The bill, which takes its name from the code name of a military defector who smuggled 53,275 photographs out of Syria showing Assad's war crimes, passed the House in January.

ABC News's Matt McGarry contributed to this report from Dubai.