Seven people have been killed after a gunman opened fire during Sunday services at a Sikh temple in Wisconsin.

Four people were shot inside the building in Milwaukee and three outside, including a gunman, who was killed by a police officer.

The 20-year veteran police officer who exchanged fire with the gunman was hit several times. He was taken to hospital and is expected to survive.

A least three other people were critically wounded.

Smiran Kaleka, the niece of one of the wounded men, said an unknown number of gunmen had walked into the kitchen of the Sikh Temple of Wisconsin in Oak Creek and opened fire.

A man who said his father had been wounded also reported that there had been multiple gunmen.

Others who were inside the building at the time have described the attack as "a very coordinated thing."

The White House has said that President Barack Obama has been informed of the shooting and is being kept regularly updated on the situation.