President Donald Trump declined on Friday to explain what he meant when he described a gathering of military leaders as "the calm before the storm," but the White House said he was not just being mischievous when he made the remark.

Pressed about what he meant by Thursday's comment, the U.S. leader declined to elaborate, telling reporters only that, "You'll find out." White House spokeswoman Sarah Sanders also declined to say what Trump meant.

When asked whether Trump was just being mischievous, Sanders denied he was just "messing with the press."

"I think we have some serious world issues here. I think that North Korea, Iran both continue to be bad actors and the president is somebody who's going to always look for ways to protect Americans," Sanders said.

Leon Panetta, a former Defence secretary and CIA director, said Trump's remarks would be something "you'd really worry about" under a previous U.S. president. But he said Trump's comments appeared to follow a pattern he'd established on Twitter.

"You begin to assume that it's more about getting attention than it is about proclaiming some kind of national policy. I don't think it's responsible...but I think in this instance we probably all should take a deep breath and try to assume he's just making a play for attention," Panetta told CNN.

"There is no indication that there is a strategy or a policy behind that statement," he added.

Trump made the "calm before the storm" comment during a photo opportunity before having dinner with U.S. military leaders and their wives. The dinner followed a meeting in which Trump and the military leaders discussed Iran, North Korea, Afghanistan and the fight against Islamic State.

The president also appeared to criticise the military leaders on Thursday for moving too slowly to provide him advice.

"Moving forward, I also expect you to provide me with a broad range of military options, when needed, at a much faster pace. I know that government bureaucracy is slow, but I am depending on you to overcome the obstacles of bureaucracy," he said.

Asked if Trump felt like military leaders were deliberately being slow to advise him, Sanders said, "Not at all."

"As you know, he's a person who like to take action and take it quickly," she said. "He wants options on the table so that he can make quick decision."

Reuters