General Kenneth McKenzie, director of the US military's Joint Staff, said Syrian President Bashar al-Assad's regime retains a "residual" chemical capability at a variety of sites across the country.

"They will have the ability to conduct limited attacks in the future," McKenzie told Pentagon reporters.

"However as they contemplate the dynamics of conducting those attacks, they have to look over their shoulder and be worried that we are looking at them, and we will have the ability to strike them again should it be necessary."

Pentagon spokeswoman Dana White said there was no indication the Assad regime was preparing to launch another chemical weapons attack.

"Assad must know the world will not tolerate the use of chemical weapons under any circumstances," she said.

On April 13, the US, Britain and France fired more than 100 cruise missiles at three Syrian sites, including a large research center in Damascus, in response to an alleged chemical attack in Douma that killed more than 40 people.

According to satellite imagery displayed by the Pentagon, the three sites were completely destroyed.

"We achieved the level of success that we wanted against those three targets," McKenzie said.

"We believe that there was probably some chlorine and possibly sarin at possibly all of the sites."

The three-star general added the Syrian regime had now returned to a "state of normalcy."

"I don't think we sought to change the strategic balance of the Syria conflict with those strikes. We sought to send a lesson that it's bad practice to gas women and children," McKenzie said.