Mattis said it's violation of UNSC resolutions.
Pentagon chief Jim Mattis said during a visit to Israel on Friday that there can be "no doubt" that Syria has retained some chemical weapons and warned President Bashar al-Assad's regime not to use them.
Mattis, speaking during a press conference with Israeli counterpart Avigdor Lieberman, declined to comment on the number of weapons Washington believes Syria has retained.
US President Donald Trump's administration carried out a strike against a Syrian airbase earlier this month over an alleged chemical attack on a rebel-held town.
"The bottom line is there can be no doubt in the international community's mind that Syria has retained chemical weapons in violation of its agreement and its statement that it had removed them all," Mattis said.
"It's a violation of the United Nations Security Council resolutions, and it's going to have to be taken up diplomatically, and they'd be ill-advised to try to use any again. We've made that very clear with our strike."
An Israeli military assessment has found that Assad's regime was still in possession of "a few tonnes" of chemical weapons, an army official confirmed.
Some Israeli media reports put the number at between one and three tonnes. Lieberman declined to comment on the assessment at Friday's press conference.
Assad, backed by his ally Russia, has strongly denied the allegation that his forces used chemical weapons against the rebel-held town of Khan Sheikhun on April 4, describing it as a "100% fabrication".
He has said repeatedly that his forces turned over all chemical weapons stockpiles in 2013, under a deal brokered by Russia to avoid threatened US military action.
The agreement was later enshrined in a UN Security Council resolution.
Addressing North Korea, Mattis called comments from Pyongyang "provocative" but added that "their word has not proven honest."
"They've not lived up to any statements they've made in the past years, decades actually, about stopping their ballistic missile and their nuclear programmes," Mattis said. "So I would just take their words at the same value we've always given them as we've learnt not to trust them."
North Korea is seeking to develop a long-range missile capable of hitting the US mainland with a nuclear warhead, and has so far staged five nuclear tests, two of them last year.
Vice President Mike Pence vowed on Wednesday that the United States would counter any North Korean attack with an "overwhelming and effective" response.
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 "Syria, Bashar al-Assad, Chemical Weapons, Russia, United States of America (USA), Jim Mattis, US Defence Secretary, North Korea,")
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