Six people were killed in an attack on a Gurudwara during Sunday morning prayers in Wisconsin by at least one gunman who was also shot dead, police said.

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.