National Guard troops are positioned outside Chicago and they could be in Memphis by Friday as the Trump administration pushes ahead with an aggressive policy toward big-city crime whether local leaders support it or not.

National Guard members from Texas had settled in at an Army Reserve centre in Illinois by early Wednesday, despite a lawsuit and vigorous opposition from Democratic elected leaders.

Their exact mission was not clear, though the Trump administration has an aggressive immigration enforcement operation in the nation's third-largest city, and protesters have frequently rallied at an immigration building outside Chicago in Broadview.

The president has called Chicago a "hell hole" of crime, although police statistics show significant drops in most crimes, including homicides.

In Memphis, Tennessee, police Chief Cerelyn Davis said a small group of commanders were already in the city, planning for the arrival of Guard troops.

Republican Gov. Bill Lee has said troops will be deputised by the US Marshals Service to "play a critical support role" for local law enforcement, though that role hasn't been precisely defined yet.