US President Donald Trump said on Tuesday he would deploy National Guard troops to fight crime in Chicago, an extraordinary effort to militarise the country's third-largest city that was likely to trigger a legal battle with local officials.

"We're going in. I didn't say when, but we're going in," Trump told reporters at the White House.

Chicago Mayor Brandon Johnson swiftly condemned the action.

"He just wants his own secret police force that will do publicity stunts whenever his poll numbers are sinking, whenever his jobs report shows a stagnating economy, whenever he needs another distraction from his failures," he told reporters.

Hours earlier, a federal judge blocked Trump's administration from using the military to fight crime in California, a ruling that does not apply to other states. But the judge said that Trump's stated desire to send troops to Chicago and other cities provided support for his ruling.

Since taking office, Trump has attempted to broaden the role of the military on US soil, which critics say is a dangerous expansion of executive authority that could spark tensions between the military and ordinary citizens.

Trump has issued a barrage of executive orders and taken other unilateral steps to achieve policy goals ranging from curtailing immigration and cutting the size of the federal workforce, to ending affirmative action and bringing elite universities and law firms to heel.

Illinois Governor JB Pritzker said after Trump spoke that he had learned from reporters that the Trump administration has "gathered Ice agents and military vehicles, and that there are more Ice agents that are on the way."

He said the Trump administration was staging Texas National Guard for deployment in Illinois, along with federal agents from Ice, Customs and Border Control, the Department of Homeland Security and other agencies.

He said Trump would position armed federal agents and stage military vehicles on federal property such as the Great Lakes Naval Base, including individuals relocated from Los Angeles.

The Republican president said he would "love to have" Pritzker, a Democrat, call and request troops, but "we're going to do it anyway."

"We have the right to do it," Trump said, adding that the federal intervention would extend to Baltimore as well.

Johnson, also a Democrat, said over the weekend that Chicago police will not collaborate with any National Guard troops or federal agents if Trump deployed them to the city as threatened.

Pritzker, whose name has also been floated as a possible 2028 presidential candidate, said the state was able to gather more details from "unauthorised patriotic officials inside the government and from well-sourced reporters" about Trump's plans to bring military personnel to the city. (Reuters)