Three police officers were killed and three injured in a shooting in Baton Rouge, Louisiana on Sunday, with at least two suspects at large, police said.

"Three law enforcement (officers) are confirmed dead, three others injured," the East Baton Rouge sheriff's office said in a statement. "One suspect is dead, law enforcement believes two others may be at large."

The shooting took place along a highway around 9:00 am (1400 GMT), the statement added.

It appears the incident began before police officers were at the scene.

"It's my understanding that they (the officers) had responded to an initial shooting incident," Casey Rayborn Hicks, public affairs officer for the sheriff's office, told local WAFB television.

Witness Brady Vancel told the station that he saw what may have been gang members shooting at each other before police arrived.

"This has nothing to do with any situation," he said, referring to controversies surrounding a series of high-profile shootings involving police that have shocked the country over the last several weeks.

A gunman killed five police officers in Dallas earlier this month during a demonstration triggered by the fatal police shooting of two African-American men whose dying moments were captured in shocking video footage that went viral online.

One of those killed was Alton Sterling, shot by a police officer in Baton Rouge two days before the Dallas attack.

The shootings helped expose deep fault lines through society, reviving long-running debates about racial prejudice and an epidemic of gun violence.

Wearing all black

During Sunday's shooting, Vancel said he saw two men running away and a third lying motionless on the ground. At least one was carrying what appeared to be an AR-15 automatic rifle amid the sound of gunfire, he said. "I don't believe police were targeted."

The races of the shooters and the police officers were not immediately clear.

WAFB broadcast video footage of police responding to the scene near a gasoline station. Multiple shots could be heard as civilian cars quickly backed away.

However, there were conflicting reports about the shooting, with WBRZ television reporting a witness describing a man dressed in black, his face covered, shooting indiscriminately.

Police are looking for the two suspected surviving shooters.

"We're not sure of anything right now," Baton Rouge police spokesman L'Jean McNeely told reporters near the scene, urging local residents to be on the alert for the suspects.

"If they're wearing all black, army fatigues, anything that's suspicious in nature," citizens should notify police, he said.

The injured police were reported to be in critical condition, undergoing treatment at a nearby trauma center.

Video posted online showed a SWAT team en route to the scene.

Unspeakable and unjustified

"This is an unspeakable and unjustified attack on all of us at a time when we need unity and healing," Governor John Bel Edwards said in a statement. "Rest assured, every resource available to the state of Louisiana will be used to ensure the perpetrators are swiftly brought to justice."

Tensions between Baton Rouge police and residents have been running high since the killing of Sterling, 37, whom officers shot while he was pinned to the ground following a scuffle in front of a convenience store where he had been selling CDs.

Last week, police arrested more than 100 protesters taking part in a demonstration against police brutality in Baton Rouge under the banner of the Black Lives Matter movement.

Sunday's shootings will also play into a debate about gun control in a country in which firearms killed some 13,286 people last year.

Last month, Democratic lawmakers, pushing for tougher gun-control laws after a massacre in a Florida gay nightclub killed 49 people, staged a virtually unprecedented 24-hour sit-in in Congress after Republicans refused to allow a vote on two widely supported measures.