Washington warned Tuesday that if Damascus were to use chemical weapons, it would respond.
"The United States is closely monitoring the situation in Idlib province, Syria, where millions of innocent civilians are under threat of an imminent Assad regime attack, backed by Russian Federation and Iran", the White House said in a statement on Tuesday.
Turkey and the USA said they were racing find a diplomatic solution to avoid "a serious massacre" in Syria's Idlib province ahead of a critical summit on Friday which is likely to decide the fate of the rebel stronghold.
A US advisor warned Thursday that there is "lots of evidence" that Syria is preparing chemical weapons as it gears up for an assault on Idlib, the country's last major rebel-held enclave, Reuters reports.
According to the UN, Idlib is home to about 2.9 million civilians, including 1.4 million displaced from other parts of Syria.
Iranian television reported that the three leaders would each have "bilateral meetings" on the sidelines of the main summit.
De Mistura cited press reports indicating that Syria has set a September 10 deadline for finding a solution before it begins an all-out offensive on the province.
Staffan de Mistura called for efforts "to avoid that the last probably major battle of the Syrian territorial conflict. ends in a bloodbath".
The UK-based Syrian Observatory for Human Rights said warplanes, which it believed were Russian, struck the southern countryside of Idlib and a village in nearby Hama province.
One of the newly sanctioned individuals, Muhammad al-Qatirji and his company Qatirji Company, maintain "strong ties to the Syrian regime and facilitates fuel trade between the regime and ISIS, including providing oil products to ISIS-controlled territory", according to information provided by the Treasury Department, which oversees and enforces sanctions.
North Korean hacker charged with Sony, WannaCry attacks
It's been a year and half since he launched a ransomware attack that hit hundreds of thousands of computers across the globe. Prosecutors said the complaint was sealed for a variety of reasons and wasn't done to prevent derailing the Singapore talks.
Seized from government forces in 2015, Idlib and surrounding areas form the last major swath of territory still in rebel hands.
The Syrian military has been urging the rebels in Idlib to surrender. Syrian government forces resumed shelling of rebels there.
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 have prompted terrified residents to flee before a fully-fledged assault is launched, the monitor said.
According to an upcoming book by reporter Bob Woodward, Trump wanted to kill al-Assad after he launched the chemical weapons on civilians a year ago.
Trump wanted to have Assad assassinated a year ago but Defense Secretary James Mattis ignored the request, according to the book "Fear: Trump in the White House".
"Let's f-king kill him!"
Any new flood of refugees toward Turkey would come at a time when Ankara is "vulnerable", he told AFP, adding it would be a "huge new burden on Turkey and would overwhelm its humanitarian capacities".
As for Trump's take on the reversal, Jeffrey said, "I am confident the president is on board with this".