Three police officers have been killed and three others wounded in another deadly shooting in the US targeting law enforcement, this time in Baton Rouge, Louisiana.

Police were responding to reports of a suspicious person armed with an assault rifle on Sunday morning. When they arrived on the scene, the gunman opened fire.

The suspect in the incident was killed as officers returned fire, and he has been named by local media as Gavin Long, a former marine sergeant who was shot dead on his 29th birthday.

One of the officers was listed as critical and another in fair condition. The status of the third wounded officer is unknown.

The shooting unfolded at 9am just one mile away from police headquarters in the area of Airline and Old Hammond highways. Roads were closed off in both directions between Goodwood Boulevard and Interstate 12.

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According to radio traffic, the first officers to arrive were unable to work out where the shooting was coming from.

Almost six minutes pass after the first shots are reported before police say they have determined the shooter's location. About 30 seconds later, someone says shots are still being fired, and officers can be heard making urgent calls for backup from an armoured personnel carrier.

President Barack Obama condemned the shooting on Sunday afternoon.

"For the second time in two weeks, police officers who put their lives on the line for ours every day were doing their job when they were killed in a cowardly and reprehensible assault," he said.

"These are attacks on public servants, on the rule of law, and on civilized society, and they have to stop."

Baton Rouge Gunman

The president added that he's given the full support of the federal government to Governor Edwards, Mayor Holden, and the Baton Rouge Police Department.

"We may not yet know the motives for this attack, but I want to be clear: there is no justification for violence against law enforcement,” he said. “None. These attacks are the work of cowards who speak for no one.”

Airline Highway has been the site of protests following the fatal police shooting of Alton Sterling on July 5. Footage of the incident, which showed at least one officer shooting Sterling at point blank range, ignited protests and outrage across the country.

In the days following Sterling's death, and another fatal police shooting of a black man in Minnesota, a gunman shot and killed five police officers in Dallas at an anti-police brutality protest.

Those shootings were the subject of a series of social media postings by Cosmo Setenpra, identified by CBS News as an alias used by the Baton Rouge shooter, Gavin Long.

Long described the Dallas shooter Micah Johnson as “one of us”, and when discussing the shootings of Sterling and Philando Castile wrote: “Power doesn’t respect weakness. Power only respects Power.”

His last post under the assumed name was at around 1am on Sunday morning, in which he wrote: “Just bc you wake up every morning doesn't mean that you're living. And just bc you shed youphysical body doesn't mean that you're dead [sic].”

Last week, police arrested four suspects deemed as offering a credible threat against law enforcement officials. One 12-year-old suspect was arrested after breaking into a store and stealing eight handguns. A second 17-year-old suspect reportedly said that he wanted to kill police officers.

Donald Trump, the presumed Republican nominee for president, mourned the fallen Baton Rouge officers in an overtly political message on Sunday.