The Pentagon is working on plans to deploy the US military to Chicago as President Donald Trump says he is cracking down on crime, homelessness and undocumented immigration, the Washington Post reported on Saturday.

The Defence Department planning, in the works for weeks, involves several options, including mobilising at least a few thousand members of the National Guard as soon as September, the Post reported.

"Chicago is a mess," Trump, a Republican, told reporters on Friday, deriding its mayor as he continued his attacks on cities run by Democratic politicians.

"And we'll straighten that one out probably next."

The Pentagon said in a statement late on Saturday: “We won’t speculate on further operations. The department is a planning organisation and is continuously working with other agency partners on plans to protect federal assets and personnel."

JB Pritzker, the Democratic governor of Illinois, which includes Chicago, said in a statement the state had received no outreach from the federal government on whether it needed assistance.

He said there was no emergency warranting a National Guard or other military deployment.

"Donald Trump is attempting to manufacture a crisis, politicise Americans who serve in uniform and continue abusing his power to distract from the pain he is causing working families," Pritzker said.

On Friday, Chicago Mayor Brandon Johnson said the city had grave concerns about the impact of any unlawful deployment of National Guard troops.

"The problem with the President's approach is that it is uncoordinated, uncalled for and unsound," said Johnson, adding that over the past year, homicides in Chicago have fallen by more than 30 percent, robberies by 35 percent and shootings by almost 40 percent.

At Trump's request last weekend, the Republican governors of three states said they were sending hundreds of National Guard troops hundreds of miles to Washington, DC. (Reuters)