TEL AVIV, Israel — The Syrian government has retained an unspecified amount of chemical weapons and dispersed its aircraft following a U.S. cruise missile attack earlier this month, Secretary of Defense Jim Mattis said Friday.

Mattis spoke alongside his Israeli counterpart, Avigdor Lieberman, as part of a tour of the Middle East, and addressed reports that Syrian President Bashar Assad still had a significant amount of chemical weapons.

“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.

Mattis did not provide any evidence to back up his claim.

On April 7, the U.S. launched dozens of cruise missiles at Shayrat airbase in Syria in response to a chemical weapon attack on Idlib province that happened three days earlier and killed nearly 100 people, including 30 children.

U.S. officials said the Syrian aircraft that conducted the attack took off from Shayrat airfield.

An unnamed Israeli military officer told reporters Wednesday that Assad still had a “few tons” of chemical weapons despite his pledge to destroy them under a 2013 agreement. It is unclear if the Israeli officer was talking about sarin gas — the same type reportedly used in the April 4 attack — or chlorine gas. Assad has regularly used chlorine during the 5-year-old war.

Sarin was last used in Syria in 2013. The attack killed hundreds and nearly saw the U.S. launch strikes on Syrian military facilities. A last minute deal, brokered in part by Russia in October 2013, instead made Syria pledge to destroy its chemical weapon stockpiles and sign the Organisation for the Prohibition of Chemical Weapons’ ban to ensure the weapons were not used in the future.

The same group confirmed that sarin, a prohibited substance under the ban, was used in the April 4 attack.

“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,” Mattis said.

Mattis also addressed reports that Syrian warplanes had relocated to Russia’s airbase in northwestern Syria to avoid future attacks following the cruise missile strikes. While he would not say where the planes had gone to, Mattis said Syrian forces had “dispersed their aircraft in recent days.”

Pentagon officials said the cruise missile strikes had destroyed roughly 20 Syrian aircraft. If true, the lost jets would moderately hamper a Syrian air force already significantly reduced after dozens of losses over the years.