After discussing Iran and North Korea with U.S. military leaders on Thursday, President Donald Trump posed for a photo with them before dinner and declared the moment "the calm before the storm."

"You guys know what this represents?" Trump said after journalists gathered in the White House state dining room to photograph him and first lady Melania Trump with the uniformed military leaders and their spouses.

"Maybe it's the calm before the storm," he said.

What storm?

"You'll find out," Trump told questioning reporters.

Classical music played in the background and tables were set in the nearby Blue Room for a fancy meal.

The White House did not immediately reply to a request to clarify Trump's remark.

Earlier in the evening, while seated with the top defence officials in the cabinet room, Trump talked about the threat from North Korea and preventing Iran from getting a nuclear weapon.

"In North Korea, our goal is denuclearisation," he said. "We cannot allow this dictatorship to threaten our nation or our allies with unimaginable loss of life. We will do what we must do to prevent that from happening. And it will be done, if necessary, believe me."

During his speech to the United Nations General Assembly last month, Trump said the United States would "totally destroy" North Korea if needed to defend itself or U.S. allies.

The president on Thursday also had tough words for Iran, saying the country had not lived up to the spirit of an agreement forged with world powers to curb its nuclear programme.

A senior administration official said on Thursday that Trump was expected to announce soon he would decertify the landmark agreement.

Trump has filled top posts within his administration with military generals, including his chief of staff, retired General John Kelly, and national security adviser, Lieutenant General H.R. McMaster. McMaster, who normally dresses in civilian clothes at the White House, wore his uniform for the meeting.

Without being specific, Trump pressed the leaders to be faster at providing him with "military options" when needed.

"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 during their cabinet room meeting.