At least eight people were killed in an incident Thursday in the US city of Indianapolis by a gunman who is believed to have then killed himself, police said.

All of the victims were found at a Fedex facility near the city's international airport where a gunman had opened fire, police spokeswoman Genae Cook told reporters.

Several others were transported to hospital, police said, but did not disclose precise numbers.

One man who said he works at the plant told local broadcaster WISH-TV he saw the gunman start shooting.

"I saw a man with a sub-machine gun of some sort, an automatic rifle, and he was firing in the open. I immediately ducked down and got scared," Jeremiah Miller said.

Police spokeswoman Genae Cook told reporters officers had responded to an "active shooter incident," adding they believe the gunman died by suicide.

Cook said authorities judged there was no longer an immediate threat to public safety.

The shooting happened at a facility close to Indianapolis international airport. Photo: Shutterstock

A Fedex spokesperson confirmed to AFP that its facility was the scene of the shooting, and said the company was cooperating with authorities.

"We are aware of the tragic shooting at our ground facility near the Indianapolis airport," the company said in a statement. 

"Safety is our top priority, and our thoughts are with all those who are affected."

The facility for the delivery company is reported to employ more than 4,000 people.

Live video showed police tape at the scene of the incident, which follows several mass shootings in recent weeks.

At the end of last month, four people, including a child, were shot dead in an office building in southern California.

On March 22, 10 people were killed in a shooting at a grocery store in Boulder, Colorado. 

That came less than a week after a man shot and killed eight people, including six women of Asian descent, at spas in Atlanta, Georgia.

Nearly 40,000 people in the United States die each year from guns, more than half of those being suicides.

The issue of gun regulation in the United States is politically fraught.

President Joe Biden this month announced six executive measures he said would help stem the gun violence crisis.

"It's an international embarrassment," the Democrat told Congress members and gun control activists at a White House ceremony.

The move was immediately attacked by Republicans, with the party's senior leader in the House of Representatives, Kevin McCarthy, warning of "unconstitutional overreach."