"The United States will continue to target those who facilitate transactions with the murderous Assad regime and support ISIS", Mnuchin said in the statement. "With millions of civilians at risk, an offensive against Idlib would be a reckless escalation".
The two men, shown arriving by plane by Iranian state television, were due to hold talks with Iranian President Hassan Rohani.
More than 3 million people live in the Idlib area and U.S. Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff, told reporters that a "human catastrophe" could unfold if the Syrian government attacks the province.
"There is lots of evidence that chemical weapons are being prepared" by Syrian government forces in Idlib, the U.S. State Department's adviser for Syria, Jim Jeffrey, said in an interview with Reuters.
According to an upcoming book by reporter Bob Woodward, Trump wanted to kill al-Assad after he launched the chemical weapons on civilians previous year.
Russian Foreign Ministry Spokeswoman Maria Zakharova said that barrels of chlorine and munitions had been delivered to Idlib, while members of the White Helmets, which had been involved in the staged chemical weapons incident in the Syrian town of Douma in April, were getting their cameras ready. President Trump has previously labelled Assad an "animal" for using the banned weapons, and called Iran and Russian Federation "responsible for backing" him.
Worldwide calls for restraint grew Wednesday for Syria and its allies, Russian Federation and Iran, to avoid a bloodbath and humanitarian disaster in Syria's Idlib province.
Turkey's President Recep Tayyip Erdogan on Wednesday urged Damascus against launching a battle to retake Idlib, the last major chunk of Syria still in opposition hands.
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It has also created border security zones in northern Syria and has several hundred troops deployed at 12 observation posts in Idlib.
Trump shifted his public tone on Syria within a matter of weeks, as the US and its allies launched missile strikes against Syria in April over a chemical attack against civilians in the city of Duoma.
"I think the last chapter of the Idlib story has not been written".
"The consequences of that are that we will shift our positions and use all of our tools to make it clear that we'll have to find ways to achieve our goals that are less reliant on the goodwill of the Russians", Jeffrey said.
Northwestern Idlib province and surrounding areas are home to about 3 million people - almost half of them civilians displaced from other parts of Syria.
Turkish President Recep Tayyip Erdogan said Friday at the meeting that an attack on Idlib would result in a disaster and massacre, and that millions of people would be moving towards the Turkish border in the case that an attack took place. That also includes an estimated 10,000 hard-core fighters, including Al-Qaeda-linked militants.