In a dark speech before hundreds of generals and admirals summoned from around the world, the Republican told them to prepare for a greater role in crackdowns on Democrat-run cities.

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 US flag at a military facility in Quantico, near Washington.

He 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 has sent troops to Los Angeles and Washington as part of what he calls a crackdown on crime and immigration. He has also ordered deployments to Memphis and Portland, which he described as a "war zone," and said Chicago would be next.

Each of those cities is run by a Democrat mayor, from Trump's rival political party.

The 79-year-old -- who traveled to make the speech as the US government hurtled towards a spending shutdown -- began by saying that under his administration the US military was now "reawakening the warrior spirit."

But his 72-minute 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.

- 'Fat generals' -

Former Fox News host-turned Defense Secretary Hegseth summoned the highly unusual meeting last week. 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 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, amid the deployments to key cities, and abroad.

- 'American muscle' -

Internationally, Trump has ordered lethal strikes on small, alleged drug boats in the Caribbean, despite questions over the legality of the attacks.

He also ordered strikes on Iranian nuclear sites and Tehran-backed Yemeni rebels.

In a sometimes rambling speech, the US president said he was "discovering American muscle" and that the country had the "strongest military anywhere in the world."

Trump has overseen a rare purge of senior officers after taking office.

In May, Hegseth ordered major cuts to the number of general and flag officers in the US military, including at least a 20 percent reduction in the number of active-duty four-star generals and admirals.

Since beginning his second term in January, Trump has also purged top officers, including chairman of the Joint Chiefs of Staff general Charles "CQ" Brown, whom he fired without explanation in February.

Other senior officers dismissed this year include the heads of the Navy and Coast Guard, the leaders of the National Security Agency and the Defense Intelligence Agency, the vice chief of staff of the Air Force, a Navy admiral assigned to NATO, and three top military lawyers.

Hegseth defended the firings on Tuesday, saying: "it's nearly impossible to change a culture with the same people who helped create -- or even benefited from -- that culture."