"We will continue to seek accountability through all available diplomatic mechanisms, including the United Nations Security Council and the Organization for the Prohibition of Chemical Weapons, for the confirmed use of chemical weapons by any party", the State Department said in a statement.
They also said that President Donald Trump is prepared to again order military action against Syrian leader Bashar al-Assad's forces if necessary to deter chemical attacks.
The Syrian regime denies having any connection to that attack and, in general, to any chemical weapons attacks that have occurred in Syria since the start of the civil war there.
Mr Trump ordered missile strikes against a Syrian air base last April in response to alleged chemical weapons attacks on rebel areas.
Syrian President Bashar al-Assad may be developing new chemical weapons in addition to keeping some of the country's stockpile that was supposed to be handed over and destroyed under a 2014 deal brokered by the United States and Russian Federation, senior US officials told reporters on Thursday. They also say it's "likely" Syria has kept a secret stockpile of chemical weapons, even after it agreed to destroy the stash in 2013.
A Civil Defence member carries a damaged canister in Ibleen village from what activists said was a chlorine gas attack, on Kansafra, Ibleen and Josef villages, Idlib countryside May 3, 2015.
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"We reserve the right to use military force to prevent or deter the use of chemical weapons", one official said, while declining to specify how serious a chemical attack would have to be to draw a fresh USA military response.
The senior US officials, speaking to journalists on the condition of anonymity because they were not authorized to speak publicly, said there did not appear to have been any "large use" of banned munitions since April.
"We do not have evidence", Mattis told reporters according to Politico.
"We reserve the right to use military force to prevent or deter the use of chemical weapons", one official told Reuters, although he did not specify the severity of an attack that would draw a renewed United States military response. Nearly all independent analysts and investigators agree the Syrian regime is the culprit of the chemical attacks.
The Syrian Arab Republic ratified the convention in 2013 following a sarin gas attack on opposition-held neighborhoods in East and West Ghouta that killed at least 1,400 people.
The Damascus suburb of around 400,000 people has been besieged by regime forces since 2013, in an attempt to strangle the rebel region into submission.