U.S. Defense Secretary Jim Mattis said on Friday that Syria had dispersed its warplanes in recent days and that it retained chemical weapons, an issue he said would have to be taken up diplomatically.

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

The Pentagon has said that the strike had damaged or destroyed about 20 percent of the Syrian military’s operational aircraft.

During a press conference alongside his Israeli counterpart, Mattis was asked whether the Syrian military had moved warplanes to a Russian base in Latakia.