The Syrian foreign ministry denied on Saturday US allegations it had used chemical weapons against rebel-held Eastern Ghouta enclave near Damascus, saying the accusations were "lies" without evidence.
"The Foreign Ministry decisively condemns U.S. groundless allegations against the Syrian leadership of chemical weapons use in Eastern Ghouta [in eastern suburbs of Damascus]", the statement read, as quoted by the Syrian state-run SANA news agency.
Mattis says it is clear that Assad's government has weaponized and used chlorine gas in the Syrian civil war.
Meanwhile US Defence Secretary James Mattis said on Friday that the Syrian government would be ill-advised to "go back to violating the chemical (weapons) convention".
Mattis told reporters at the Pentagon on Friday that the U.S.is looking for evidence to support the claims.
However, another official said that the Trump administration hopes that stepped-up worldwide sanctions and diplomatic pressure against Damascus will help rein in Syria's chemical weapons program.
"We are even more concerned about the possibility of sarin use, (but) I don't have the evidence,"Mattis said".
However, the Syrian Negotiations Commission (SNC), an opposition group, said chemical weapons had been used by the government of Bashar al-Assad in Ghouta.
The French government announced Tuesday that it is freezing the assets of companies that help furnish material to the Syrian Scientific Studies and Research Center, a Syrian lab accused of producing chemical weapons.
In his remarks Friday, Mattis alluded to the April attack, saying, "So they'd be ill-advised to go back to violating" the worldwide prohibition on the use of chemical weapons.
Syrian activists say civilians have suffered chlorine gas poisoning during an attack on the rebel-held town of Saraqeb in Idlib province.
However, Syria and Russian Federation had rejected the United States claims, suggesting that militants might have detonated the chemical weapons on the ground.