The Syrian government denies using chemical weapons; says extremists have used chemical weapons in the past.
"The foreign ministry condemns the false claims by the United States that the Syrian government used chemical weapons in Eastern Ghouta", a foreign ministry source said, cited by Syrian state news agency SANA.
Assad's forces have continued occasional use of chemical weapons in smaller amounts since a deadly attack last April that drew a United States missile strike on a Syrian airbase, the officials told reporters in a briefing.
Mattis also said on Friday that the US government was concerned about the Syrian government's use of sarin gas, but acknowledged that he did not have evidence to prove that it was using the nerve agent.
The attack comes a day after Syria's former al-Qaeda affiliate claimed responsibility for the downing of a Russian warplane as it flew low over Saraqeb in northern Syria, apparently using a surface-to-air missile.
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Look, I think we all go through our life and sometimes we say things we shouldn't say or make mistakes, and that happens. The radio station called Reimer's on-air remarks "mean-spirited" and "indefensible" in a statement released on Monday.
He said that non-governmental organizations and fighters on the ground in Syria had reported sarin use, and that the US was looking into the matter.
President Donald Trump has not ruled out additional military action to deter attacks or punish Assad, administration officials said earlier this week, although they did not suggest any action was imminent.
Mattis made the remarks after senior USA officials said Trump was prepared to order more strikes amid reports that the Syrian government is developing new types of chemical weapons.
"You all have seen how we reacted to that, so the Syrian government would be ill-advised to go back to violating the chemical convention", Mattis said. The officials say the militants are using shells or improvised explosive devices to deliver the chemicals.
In 2013, Syria said it would give up all of its chemical weapons. Make a donation and help pay for our journalism.