The US on Tuesday said it had intelligence on active preparations at a Syrian airfield, proving a possible chemical attack. The White House warned Syrian regime leader Bashar al Assad that he and his military would "pay a heavy price."
The Syrian regime denied the Trump administration's allegation that it is preparing for another chemical weapons attack.
On Monday, in a statement, the White House Press Secretary Sean Spicer said the US "has identified potential preparations for another chemical weapons attack by the Assad regime that would likely result in the mass murder of civilians, including innocent children."
On Tuesday, the Pentagon said what looked like active preparations for a chemical attack had been seen at Shayrat, in south-east of Homs.
In response, Ali Haider, regime minister of national reconciliation of Syria, said the US allegation is unacceptable and that the Syrian government has no chemical weapons.
He also accused the White House of starting a diplomatic war at the United Nations.
"Took the warning seriously"
Syria appears to have heeded a US warning against staging any new chemical weapons attack as no such action has been launched, US Defense Secretary Jim Mattis said on Wednesday.
"It appears that they took the warning seriously," Mattis told reporters flying with him to Brussels for a meeting of NATO defence ministers. "They didn't do it."
Asked whether he believed Syrian regime leader Bashar al Assad's forces had called off any such strike completely, Mattis said: "I think you better ask Assad about that."
Daesh attacks Raqqa
Daesh militants launched attacks targeting Syrian Democratic Forces (SDF) in an attempt to retrieve the areas they lost, the Syrian Observatory for Human Rights reported.
A senior SDF military member said the militants conducted an attack using motorbike and car bombs, but that the coalition destroyed the bombs and killed a number of Daesh militants.
Clashes are going on in the fight against Daesh on several fronts on the western side of Raqqa, according to the observatory.
Daesh militants are trying to retrieve areas they lost by launching car bomb attacks, the observatory said.
Daesh has been losing territory across northern Syria over the past 18 months to SDF to opposition forces backed by Turkey and to the Syrian regime army.
The US-backed SDF, a coalition of Kurdish and Arab groups, began its assault on Raqqa this month after a long campaign to isolate Daesh inside the city.
More air strikes
The observatory said an air strike early on Wednesday killed at least 30 civilians and injured dozens more in a village held by Daesh in eastern Syria.
The strike, in Al Dablan, about 20 kilometres south-east of Al Mayadin on the west bank of the Euphrates, is the second in 48 hours that the observatory says has killed dozens of people.
The identity of the jets that carried out the air strike was not known, the Britain-based war monitor said.
On Monday, a coalition air strike in Al Mayadin hit a building used by Daesh as a prison, killing 57 people, the observatory said on Tuesday.
The coalition did not immediately respond to a request for comment on whether its jets carried out Wednesday's strike in Al Dablan.
On Tuesday, it said it had hit targets in Al Mayadin the previous day on a mission "meticulously planned" to avoid harming civilians.
Hard to avoid civilian casualties
The coalition says it takes great pains to avoid harming or killing civilians and investigates all reports that it has done so.
The Syrian regime and Russia also deny targeting civilians.
Syria's army and its allies are pushing through the desert to relieve their own besieged Euphrates enclave in the city of Deir Ezzor, 65 kilometres north-east of Al Dablan. US intelligence officials have said Daesh has relocated its leadership to Al Mayadin.