US Defense Secretary Jim Mattis said on Friday that Syria still possesses banned chemical weapons in violation of an agreement to get rid of its entire stockpile.

Mattis, speaking during a press conference with Israeli counterpart Avigdor Lieberman in Tel Aviv, declined to comment on the number of weapons Washington believes Syria has retained.

“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. There is no longer any doubt,” he told reporters.

When asked whether the Syrian regime had moved its combat aircraft to a Russian base in Latakia, Mattis said: “They have dispersed their aircraft, no doubt. They have dispersed their aircraft in recent days.”

The implication is that Syria may be concerned about additional US strikes following the cruise missile attack earlier this month in retaliation for alleged Syrian use of sarin gas.

The United States launched dozens of missiles against a Syrian air base in response to a chemical attack that killed 90 people, including 30 children. It says the Syrian regime launched the attack from the Shayrat air base.

Mattis also reiterated that the United States believed Syria had retained some chemical weapons.

“The bottom line is, I can say authoritatively they have retained some (chemical weapons). It’s a violation of the United Nations Security Council resolutions, and it’s going to have to be taken up diplomatically,” he said.

Head of Syrian regime 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 percent 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.