Speaking to reporters, Jim Jeffrey, who was appointed in August as the State Department's Special Representative for Syrian Engagement, said Thursday that any such use of chemical weapons against the last rebel stronghold would be a "reckless escalation" of the conflict.
According to an upcoming book by reporter Bob Woodward, Trump wanted to kill al-Assad after he launched the chemical weapons on civilians past year.
The Russian Defense Ministry has published footage showing how its military jets use high precision weapons to knock out what it claims are the locations of Jabhat an-Nusra terrorists in Syria's Idlib province.
The attacks come amid repeated warnings from the USA against an all-out assault by Syria's Russian-backed forces to retake Idlib, the last major area held by the armed opposition in Syria.
Syrian President Bashar al-Assad has almost quashed a seven-year-long uprising and has retaken much of the territory his forces lost to the rebellion.
The rebels expect a major humanitarian crisis, a large wave of displacement and heavy casualties if a Russian-backed offensive takes place, spokesman Najib al-Mustafa said. Idlib and its surroundings are now the only significant area where armed opposition to Damascus remains.
Turkish President Recep Tayyip Erdogan warned that dropping bombs and missiles on Syria's last rebel-held province of Idlib could cause a "massacre", in remarks published Wednesday.
Jeffrey said the U.S.is taking a tougher stance and will not tolerate an all-out attack on the region.
Jeffrey said the new United States approach is more suspicious of Russian Federation amid "growing skepticism" that Moscow is either willing or capable of assisting the U.S. with its goal of pushing "all Iranian-commanded forces from the entirety of Syria".
The U.S. Treasury said one individual, Muhammad al-Qatirji, and his firm are both tied to ISIS.
"Our cooperation with Russian Federation now is very important".
He called on all militants in Syria to disarm and seek a peaceful end to the conflict.
Though President Donald Trump has previously expressed a desire to pull USA forces out of Syria, Jeffrey said the President "is on board" with the new approach.
The US will chair a UN Security Council meeting Friday about the possible offensive.
Tehran and Moscow have helped Assad turn the course of the war against an array of opponents ranging from Western-backed rebels to Islamist militants, while Turkey is a leading opposition supporter and has troops in the country.
France has invited the United States, Jordan, Egypt, Saudi Arabia, Germany and Britain for talks on the sidelines of the UN meeting to discuss Syria, Jeffrey said. He said Assad has "no future as a ruler" of Syria but that it was not Washington's job to get rid of him.
On Monday, President Trump joined a growing chorus of voices warning the Syrian government against a full scale assault, which he said could trigger a "human tragedy".