Democratic state politician Melissa Hortman and her husband have been killed in what appears to be a "politically motivated assassination," Minnesota Governor Tim Walz says.

Democratic state lawmaker John Hoffman and his wife were also shot, and Walz says he is "cautiously optimistic" of their recovery.

US President Donald Trump says the shootings appear to be a "targeted attack against state lawmakers," adding such "horrific violence will not be tolerated".

The suspect in the Hortman shooting is believed to have impersonated a police officer and escaped after engaging fire with officers, police say.

The shootings took place in Champlin and Brooklyn Park, two neighbouring cities near Minneapolis - a shelter-in-place order has been issued by Brooklyn Park police.