The US Defence Secretary James Mattis has said there can be "no doubt" that Syria retains chemical weapons, BBC reports.
Gen Mattis also said the Syrian government had moved its warplanes in recent days, speaking during a visit to Israel.
He said Syria would be "ill-advised" to try to use chemical weapons again, after the alleged attack on 4 April which killed more than 80 people.
Syrian President Bashar al-Assad says the attack claims are 100% fabricated.
The US responded to the Khan Sheikhoun attack by launching air strikes on a Syrian military airfield.