TEL AVIV (AFP) - US Defence Secretary Jim Mattis said during a visit to Israel on Friday (April 21) 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 assessment has found that Assad's regime was still in possession of "a few tonnes" of chemical weapons, a military official confirmed.

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 per cent 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.