A gunman killed at least six people and critically injured three at a Sikh temple in northern United States before police shot him dead.

Law enforcement personnel walk near the Sikh Temple of Wisconsin where at least one gunman fired upon people at a service on August, 5, 2012 Oak Creek, Wisconsin. At least six people were killed when a shooter, who was later shot dead by a police officer, opened fire on congregants in the Milwaukee suburb. [Xinhua]

Four bodies were found inside the temple in Oak Creek, Wisconsin, while three others, including that of the suspected gunman, were found outside, local police chief Bradley Wentlandt told reporters.

The gunman ambushed one of the first officers to arrive at the temple as the officer tended to a victim outside before a second officer shot the suspect dead in a shootout, authorities said.

The police offer was in critical condition along with two other victims at a local hospital Sunday night, police said. He had a surgery and was expected to survive.

Wisconsin police are treating the shooting as a "domestic terrorist type incident." Several dozen people had gathered at the temple for services Sunday morning, according to witnesses. No further details about the gunman's identity or motives for the rampage were immediately available.

The tragedy came little more than two weeks after a gunman opened fire at a movie theater in Colorado, killing 12 and wounding 59 others, in one of the worst shooting rampages in the United States.

President Barack Obama denounced the Wisconsin shooting as a "senseless act of violence" on Sunday and sent messages of condolences for the victims.

He called Wisconsin Governor Scott Walker, Oak Creek Mayor Steve Scaffidi, and trustees of the Sikh temple Sunday afternoon, expressing his condolences, the White House said in a statement.

The president was informed by administration officials that the situation at the Sikh temple was under control, and that "the lone gunmen was killed by an Oak Creek police officer," the statement added.

In a earlier statement, Obama said he was "deeply saddened" by the tragic incident.

"Our hearts go out to the families and friends of those who were killed and wounded," Obama said, pledging his administration's commitment to fully investigate the shooting.