US President Donald Trump said on Tuesday the military should use US cities as training grounds for a "war from within," in a speech to a rare meeting of top officers.

Republican Trump told hundreds of generals and admirals summoned from around the world to be ready for a greater role in crackdowns on Democrat-run cities, including Chicago.

The assembled top brass were separately warned by Pentagon chief Pete Hegseth of a different challenge, as he vowed to eliminate "fat generals" and to roll back what he called "decades of decay."

"I told Pete, we should use some of these dangerous cities as training grounds for our military," Trump said in front of a huge American flag at a military facility in Quantico, Virginia.

Trump added that "we're going to straighten them out one by one, and this is going to be a major part for some of the people in this room. That's a war too – it's a war from within."

Trump began his speech by railing against so-called "woke" practices in the US military, saying that under his administration, it was now "reawakening the warrior spirit."

His hour-long address then took on an even more overtly political tone, in a break with previous presidents who have tended to avoid domestic politics when addressing troops.

Former Fox News host-turned Defence Secretary Hegseth summoned the meeting last week before Trump then announced that he would also speak.

Speculation had swirled about the purpose of gathering the whole US top brass in one place, with talk of a major military announcement.

But in the end it was largely a doubling down on restoring what Iraq war veteran Hegseth called the "military ethos."

Striding the stage, Hegseth told all ranks they must now take a physical fitness test twice a year. "It's completely unacceptable to see fat generals and admirals in the halls of the Pentagon," he said.

Hegseth also insisted on "grooming standards" including short hair and shaving, adding: "If you want a beard you can join special forces. If not, then shave. We don't have a military full of Nordic pagans."

He also declared an end to "ideological garbage," citing concerns over climate change, bullying, "toxic" leaders and promotions based on race or gender as examples.

The speeches by Trump and Hegseth came as the US military faces controversy both at home and abroad, with Trump deploying troops in Los Angeles and Washington, and shortly in Portland, Oregon and Memphis, Tennessee. (AFP)