Chicago: At least seven people were shot dead, including a suspected gunman, during a shooting at a Sikh temple in Wisconsin, local police said.

Greenfield police Chief Brad Wentlandt said authorities do not believe a second shooter is hiding inside the Sikh Temple of Wisconsin in Oak Creek outside Milwaukee, contrary to some media reports.

"At this time, we have not identified any additional gunmen," Wentlandt told reporters, stressing that the situation was "extremely fluid" and that law enforcement agents were still in the process of securing the scene and accounting for all the victims and witnesses.

Four people were killed inside the temple and three others outside. Wentlandt said earlier that one suspect had been "put down" outside the temple after shooting a police officer. "An officer arrived on the scene, engaged the shooter and was shot multiple times," Wentlandt told reporters. "That shooter was put down." He said the officer was being treated at a local hospital and was expected to survive.

Three adult men in critical condition were being treated at Froedtert Hospital, a spokeswoman told AFP.

"We've heard the scene is still unfolding, so we're prepared for more" victims, the hospital's chief medical officer Lee Biblo told CNN.

Police tactical units were on the scene, along with officers from multiple law enforcement agencies, including the Federal Bureau of Investigation.

The president of the temple, Satwant Kaleka, was shot and was taken to a hospital, according to The Milwaukee Journal Sentinel. It said there were believed to be as many as 20 to 30 victims.

Two shooters with children they may be holding as hostages were still inside the Sikh Temple of Wisconsin in Oak Creek outside Milwaukee, according to the newspaper, though police would not confirm the report.

It said the head priest of the temple, which had a congregation of 350-400 people when it was formally established in 1999, was locked inside a restroom with a cell phone.

Witness Suni Singh said up to four gunmen may be hiding inside the temple since the shooting took place around 10:30 am (1530 GMT).

"My friend called and said 'I heard the shot, and two people falling down in the parking lot.' He saw the shooter reloading the gun again," Singh told Newsradio 620 WTMJ.