At least 20 people were also injured when at least one shooter opened fire indiscriminately amid reports that 12 children were taken as hostages at the Sikh shrine.
Greenfield Police Chief Bradley Wentlandt said four people were gunned down inside the Gurudwara while three including the shooter died outside the shrine. The identity of the victims were immediately not known.
Police said they have not yet identified if there was any additional shooter. Preliminary reports said that at least three gunmen were involved in the attack.
According to Satwant's sister-in-law Parmindar Kaur, about 25 people are believed to have been injured. A Sikh priest had specially come from India to address the gathering on Sunday.
The critically injured have been admitted to Froedtert hospital.
The shooting at Oak Creek, just south of Wisconsin's biggest city Milwaukee, occurred at around 11 am (local time), the Milwaukee- Wisconsin Journal Sentinel reported. Oak Creek is about 1,250 km from New York.
About 300 to 400 people were reported to be at the Gurudwara, built about 6-7 years ago, and is spread over an expansive area.
Initial reports said a witness to the shooting told law enforcement agents that one shooter was of a heavy build, bald white male wearing a sleeveless T-shirt.
US President Barack Obama was also briefed by his counter-terrorism advisor John Brennan on the situation and the White House is seized of the matter, US officials said.
The shooter was last seen with two handguns.
"The people who started shooting are three white people," said Gurcharan Grewal, President Sikh Society.
A police spokesman said a law enforcement officer was shot multiple times by the unidentified shooter, who was "put down". It was not immediately known if the shooter was injured or killed.
"We do not know if there are additional shooters at the temple," he said. The officer has been transported to hospital and was expected to survive.
The spokesman said tactical operations are ongoing and that the situation was "very fluid".
Children taken hostage
The Milwaukee Sentinel reported that two shooters are possibly still inside the Gurudwara with children as hostages. Being a Sunday, the Gurudwara was likely to have been crowded.
Media reports said about 12 children have been held hostages. The president of the temple is also reportedly shot and has been rushed to the hospital, they said.
Indian Ambassador Nirupama Rao described the shooting as extremely tragic and said the Embassy was in touch with the National Security Council at White House. The Consul General at Chicago is deputing an official to the spot.
Obama takes stock of situation
US President Barack Obama has reviewed the security situation with his top national security aides following the tragic shooting in a Sikh Gurudwara in Wisconsin and also called on Gurudwara trustee to offer his condolences.
Obama called Wisconsin Governor Scott Walker, Oak Creek Mayor Steve Scaffidi and trustee of the Gurudwara Charanjeet Singh to express his condolences for the lives lost and his concern for those who were injured.
"Michelle and I were deeply saddened to learn of the shooting that tragically took so many lives in Wisconsin, Obama said in a statement.
"As we mourn this loss which took place at a house of worship, we are reminded how much our country has been enriched by Sikhs, who are a part of our broader American family," Obama said.
He also directed that the federal government should assist as appropriate in the investigation into the shooting.
The President said that he wanted to make sure that as we denounce this senseless act of violence, we also underscore how much our country has been enriched by our Sikh Community, who are an integral part of our broader American family," the White House said.