Syria also says that reports by Western-backed media outlets about Damascus using chemical weapons "is a new version of US and Western desperate intentions to create" an excuses to attack Syria.
The officials also say the Islamic State group keeps using chemical weapons such as sulfur mustard and chlorine.
Mattis told reporters at the Pentagon on Friday that the U.S.is looking for evidence to support the claims.
They emphasized that the United States was seeking a new way to hold chemical weapons-users accountable and wanted cooperation from Russia, Assad's patron, in pressuring him to end the attacks.
Mattis says it is clear that Assad's government has weaponized and used chlorine gas in the Syrian civil war.
The US launched airstrikes against a Syrian government military base last April after more than 80 people were killed in a sarin gas attack against a town held by the Syrian opposition.
Amazon, Berkshire Hathaway and JPMorgan Chase Want To Change Healthcare
It is interesting to note that one of the largest US reinsurance companies, Gen Re, is at the core of Berkshire Hathaway's empire. Intel, notes American Benefits Council president James Klein, has been a leader in direct contracting with healthcare providers.
He says the U.S.is "even more concerned" about the possible use of the nerve agent sarin.
The ministry also accused the USA of fabricating the claims and of backing "terrorist groups" responsible for chemical attacks in the region.
"They'd be ill-advised to go back to violating the chemical convention", Mattis said Friday, referring to the Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction.
The Syrian American Medical Society said its hospitals in the area treated 11 patients for chlorine gas poisoning, according to AP.
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.
In June previous year, the fact finding team of Organization for the Prohibition of Chemical Weapons (OPCW), published the results of its probe of the April 4 incident confirming that the nerve gas sarin was used in Khan Shaykhun incident.
- Trump prepared to move on Middle East peace plan without Palestinians
- Kansas City Chiefs players make it clear they'll miss Alex Smith
- Korean women's ice hockey team lose Pyeongchang 2018 warm-up game
- Goodell: No change expected with Redskins' name
- Middle School Shooting Was Accidental — LAPD
- Exxon to step up oil production in NM's Permian Basin
- Tariq Ramadan: French police question Muslim scholar over rape claims
- Rapper DMX Jailed After Failing Drug Arrests
- Sanctions seem to be taking a toll on North Korea's economy
- Everton fans delighted with defender vs Leicester: "Absolute legend", "What a man"