Chattanooga Mayor Andy Berke told a news conference: "We had someone viciously attack at two different locations people who proudly serve our country."

He added: "We do know that we have four individuals who were killed that were victims.

"We know that the shooter was killed at the scene as well.

"Also one Chattanooga police officer was shot in the ankle.

"There were some other individuals who were shot and have also been treated."

He said the attack was "incomprehensible" and a "nightmare".

Chattanooga prosecutor Bill Killian said it was being treated as "an act of domestic terrorism", and the FBI had taken over the investigation.

Police Chief Fred Fletcher told reporters the gunman had "brazenly and brutally" targeted the facilities.

Witnesses described seeing a white man armed with a high-powered rifle, driving a silver Mustang.

FBI spokesman Ed Reinhold said a naval recruiting office on the city's Lee Highway was fired upon first.

He said the Marines were killed at the Naval Reserve Center, across town on Amnicola Highway.

The gunman died at the Naval Reserve Center, but it is not yet clear if he was shot or took his own life.

Mr Reinhold said the attack began at around 10:45am and lasted for about 35 minutes.

He said it involved a single attacker and there had been no indication of an imminent attack.

Marilyn Hutcheson, who works across the road from the Amnicola Highway site, said she heard bullets fly at about 11am.

"I couldn't even begin to tell you how many," she said. "It was rapid fire, like pow, pow, pow, pow, pow, so quickly.

"The next thing I knew, there were police cars coming from every direction."

President Barack Obama has been briefed on the incident, said the White House.

Two colleges and a shopping mall in the area also issued lockdowns, which turned out to be false alerts.