"The United States will continue to target those who facilitate transactions with the murderous Assad regime and support ISIS", Mnuchin said in the statement. Turkey has accepted 3.5 million refugees from Syria since the start of the war in 2011.
The summit between Turkish President Recep Tayyip Erdogan, Russian President Vladimir Putin and Iranian President Hassan Rouhani may determine whether diplomacy halts any military action.
The entire council is scheduled to meet Friday to discuss the crisis, while at the same time, the presidents of Turkey, Russia and Iran are planning to hold a summit in Tehran.
During the trilateral summit, Erdoğan, Putin and Rouhani will discuss joint efforts as part of the Astana process and political efforts for finding a lasting solution to the Syrian conflict.
Seized from government forces in 2015, Idlib and adjacent areas form the final major chunk of Syrian territory still under opposition control.
Nearly three million people live in Idlib and adjacent rebel-held areas, half of whom have already been displaced from other parts of the country, the United Nations says.
The regime holds a small southeastern sliver. Syria's conflict has killed hundreds of thousands and displaced a staggering 11 million people both inside and outside the country.
Many of those living in Idlib are rebels and civilians who were bussed out of their hometowns in other parts of the country that have come back under regime control.
'I couldn't get air' - Roger Federer complains of humidity after shock defeat
The Australian is ranked No 55 in the world and for his heroics will be rewarded by a quarters clash against Novak Djokovic . The Spaniard will be at that stage for the second time at the US Open, five years after her other run to that round.
"I don't think that either of these countries, or really anyone, has an interest in doing something which is genuinely destabilising to Turkey", he added.
A Syrian woman sits on a straw mat next to children outside a tent at a camp for displaced civilians fleeing from advancing government forces near the village of Sarman in the rebel-held northwestern Idlib province, September 1, 2018.
As Putin and Erdogan were arriving in Tehran, the Syrian Observatory for Human Rights said air strikes were targeting positions belonging to rebel groups in the northern Hama and southern Idlib provinces. The attacks were launched with high precision weapons, it has been stated.
On Wednesday, regime artillery and rocket fire targeted several areas in the province including the HTS-held town of Jisr al-Shughur, according to the Britain-based Observatory, which relies on a network of sources inside Syria.
Syrian President Bashar al-Assad has massed his army and allied forces on the front lines in the northwest, and Russian planes have joined his bombardment of rebels there, in a prelude to a possible assault. Moscow, however, has called Idlib a "nest of terrorists", the word it uses to refer to the rebels.
Just a day earlier, Peskov had slammed Idlib as a "pocket of terrorism".
"Our cooperation with Russian Federation now is very important".
Russian presidential aide Yuri Ushakov said the leaders would focus on the situation in Idlib, and the expected chemical provocation in the area.
"We are fighting for peace".