A British-based war monitor said Wednesday it had no evidence to suggest the Syrian army had carried out a new chemical attack despite Washington's announcement it had suspicions.
"We have no proof at all of the attack," Rami Abdul Rahman, head of the Syrian Observatory for Human Rights, told AFP.
"We have not documented any chemical attack in the mountains of Latakia," he said.
The northern mountains are the only part of Latakia province, on Syria's Mediterraean coast, that are no firmly in the han ds of the government.
They are a major prize in fighting that has flared up in northwestern Syria in recent weeks between pro-government forces and the jihadists they have been used as a launchpad for rocket and drone attacks on the Hmeimim air base of the government's main ally Russia.
The Hayat Tahrir al-Sham alliance led by Syria's former Al-Qaeda affiliate accused government forces on Sunday of launching a chlorine gas attack on its fighters in the north of Latakia province.
The Syrian army dismissed the reports as a fabrication, a military source told the pro-government Al-Watan newspaper.
But the US State Department said on Tuesday it was assessing indications that the government of president Bashar al-Assad used chemical weapons on Sunday.
"We are still gathering information on this incident, but we repeat our warning that if the Assad regime uses chemical weapons, the United States and our allies will respond quickly and appropriately," State Department spokeswoman Morgan Ortagus said in a statement.
The head of Observatory said that only jihadists were present at the site of Sunday's alleged attack, making it nearly impossible to objectively confirm the incident.
"There were no civilians in the area," Abdel Rahman said.
White Helmets rescue volunteers, who have reported past chemical attacks in rebel-held areas of Syria, told AFP Wednesday that they had no information on the purported gas attack.
International inspectors say Assad's forces have carried out a series of chemical attacks during the Syrian civil war, which has killed more than 370,000 people and displaced millions since it started in 2011.