US President Donald Trump has told his top generals that America faces an “enemy from within”, in a pitch that seemed to suggest using the military against citizens.

Hundreds of top commanders were summoned back to the US from around the world to hear from Trump and Defence Secretary Pete Hegseth.

In a rambling speech on Tuesday (local time) at the US Marine base in Virginia, Trump said US cities should be used as “training grounds” for the army, in reference to him deploying troops to cities like Portland, Oregon.

He appeared to be trying to recruit the military to tackle the “enemy from within”,  and named threats such as the Democrats, the “radical left” and the media.

“We’re under invasion from within,” Trump said. “No different than a foreign enemy, but more difficult in many ways because they don’t wear uniforms.”

“In our inner cities – which we’re going to be talking about because it’s a big part of war now. It’s a big part of war.”

Trump said: “We should use some of these dangerous cities as training grounds for our military.”

“San Francisco, Chicago, New York, Los Angeles. They’re very unsafe places. And we’re gonna straighten them out one by one.

“This is gonna be a major part for some of the people in this room. It’s a war, too. It’s a war from within.”

The auditorium was filled with senior uniformed officials, seated in front of a stage with a large US flag, a lectern and boards that read: “Strength Service America”.

As Trump spoke, some officers sat expressionless, while the room was far quieter than the crowds at his usual gatherings.

One naval officer took notes.

US military senior leadership listen as Donald Trump speaks at Quantico. Photo: AAP

‘Fat and woke’

Hegseth also spoke and criticised “fat generals” and diversity initiatives that he said had led to decades of decay in the military.

He told commanders they should resign if they didn’t support his agenda.

“Foolish and reckless political leaders set the wrong compass heading and we lost our way. We became the ‘Woke Department’,” Hegseth said as he kicked off the event in Quantico.

“But not anymore.”

Hegseth defended his firings of flag officers. He said the officers he relieved were part of a broken culture.

He promised sweeping changes to how discrimination complaints are handled and accusations of wrongdoing are investigated at the Pentagon, saying the current system has top brass walking on “eggshells”.

Hegseth announced directives for troops that include “gender-neutral” or “male-level” standards for physical fitness.

“But when it comes to any job that requires physical power to perform in combat, those physical standards must be high and gender neutral,” he said.

“If women can make it — excellent. If not, it is what it is. If that means no women qualify for some combat jobs, so be it. That is not the intent, but it could be the result.”

Meetings between top military brass and civilian leaders are nothing new. But Tuesday’s gathering had fuelled intense speculation about the summit’s purpose given the haste with which it was called and the mystery surrounding it.

“If the words I’m speaking today are making your hearts sink, then you should do the honourable thing and resign,” Hegseth said.

“I know the overwhelming majority of you feel the opposite. These words make your hearts full.”

Hegseth criticised the look of overweight troops, saying: “It’s completely unacceptable to see fat generals and admirals in the halls of the Pentagon.”

He said all fitness tests would be set to male benchmarks only and emphasised the importance of grooming standards.

“The era of unprofessional appearance is over. No more beardos,” Hegseth told the audience, which sat in silence.

Trump had earlier called the meeting an “esprit de corps”.

Taking the stage after Hegseth, he spoke warmly of the military in sometimes-wandering remarks that touched on issues such as the prowess of US nuclear submarines but also included criticism of the media, former president Joe Biden and Venezuela.

“I am with you. I support you, and as President, I have your backs 100 per cent,” Trump said.

He left after his speech. There did not appear to be any one-on-one meeting with the assembled military leaders.

It is unclear how much the event cost.

-with AAP