The Catholic Bishop of Baton Rouge has called for a week of prayer and fasting following the fatal shooting of three police officers on Sunday.

"Words cannot express the emotions we feel for those who have lost loved ones in the tragic events of this day. Their entire lives have been unexpectedly and terribly turned upside down," Bishop Robert Muench said in a statement.

"In visiting this afternoon with two of the families affected by these shootings, Fr Tom Ranzino and I shared prayer and support in the midst of their shock, horror and grief. Prayer is a powerful path to follow when tragedy happens, but even the most devout of us sometime question: 'What good could come of this?' Only the Word of God has the answer to the questions that shake our faith: The answer is our Lord Jesus Christ. In Jesus, hope ultimately triumphs over despair; love ultimately triumphs over hate; and resurrection ultimately triumphs over death.

"Standing firmly on the pillars of these eternal truths, we look to his words of promise in the Sermon on the Mount, and we recall two beatitudes that speak to the hope we should hold, especially today: 'Blessed are the peacemakers, for they shall be called children of God,' and 'Blessed are they who mourn, for they will be comforted' (Matthew 5:9,4). We renew our call for a diocesan-wide week of prayer and fasting as we reflect on the events of the last several days, and as we work toward a lasting peace in our communities."

A former US Marine sergeant opened fire on police in Baton Rouge, Lousiana yesterday, killing three officers. The attack came nearly two weeks after the fatal police shooting of a black man there sparked nationwide protests, one of them shattered by the massacre of five Dallas policemen.

The Baton Rouge suspect, who has been identfied as 29-year-old Gavin Long, was himself shot to death minutes later in a gunfight with police who converged on the scene of a confrontation that Mayor Kip Holden said began as an "ambush-style" attack on officers.

Two Baton Rouge Police Department officers and one sheriff's deputy were killed, and one sheriff's deputy was critically wounded. Another police officer and one other deputy suffered less severe wounds and were expected to survive.

Colonel Mike Edmonson, superintendent of the Louisiana State Police, told a news conference the gunman was believed to have acted alone, contrary to early reports that police may have been looking for other shooters.

Sunday's bloodshed followed days of unrest over the police killings of two black men under questionable circumstances earlier this month – Alton Sterling, 37, in Baton Rouge on July 5, and Philando Castile, 32, near St Paul, Minnesota, on July 6.

Long had posted videos online in which he urged African Americans to "fight back" against unjust treatment by the US police force.

A YouTube video posted on July 10 showed Long saying he was fed up with mistreatment of blacks and suggesting that only violence and financial pressure would bring about change. He also said he was speaking from Dallas, where he had gone to join protests.

"It's only fighting back or money. That's all they care about," he said to the camera. "Revenue and blood, revenue and blood, revenue and blood."

In a separate video, he insists that should "anything happen" to him, he wanted his viewers to know he was "not affiliated" with any particular movement or group.

"I'm affiliated with the spirit of justice, nothing more nothing less," he said. "I thought my own thoughts, I made my own decisions." 

According to Long's military record, released by the Pentagon, he served in the Marines from August 2005 until August 2010, achieving the rank of sergeant. Listed as a data network specialist, he was deployed to Iraq from June 2008 until January 2009, earning several medals and commendations.

Authorities declined to offer a possible motive for Sunday's attack in Louisiana's capital, a city with a long history of distrust between African-Americans and law enforcement that was further inflamed by Sterling's slaying.

A  government source, speaking on condition of anonymity, said investigators had reason to believe an emergency 911 call may have been used to lure Baton Rouge police into harm's way.

Edmonson said several officers came under fire as police were responding to a report of a man dressed in black standing behind a store holding a rifle shortly before 9am.

Chaotic moments caught on video

In the ensuing pandemonium caught on a recording of emergency radio traffic, police are repeatedly heard reporting: "Officer down" and "deputy down" as officers swarmed the area searching for, and ultimately confronting, the gunman.

The episode was over in about eight minutes, according to Edmonson's account. At least one of the three officers killed was known to be black.

President Barack Obama condemned the attack, vowed that justice would be done and urged Americans to focus on rhetoric and actions that united the country, rather than divided it.

"We need to temper our words and open our hearts, all of us," he said in televised remarks from the White House.

Louisiana Governor John Bel Edwards called the shootings an "unspeakable, heinous attack" that served no purpose.

"There simply is no place for more violence. That doesn't help anyone, it doesn't further the conversation, it doesn't address any injustice, perceived or real. It is just an injustice in and of itself," he told reporters in Baton Rouge.

Obama has sought to balance concerns about police abuses, largely against African-Americans, while paying tribute to fallen officers.

He attended a memorial service last week for the five Dallas policemen killed by a black former US soldier who opened fire July 7 at the end of an otherwise peaceful protest denouncing the Sterling and Castile slayings.

Those two killings and the reprisal attack on Dallas police by a suspect who embraced militant black nationalism renewed tension over racial justice and gun violence, just as America's presidential campaign was kicking into high gear. The Dallas gunman, Micah Johnson, 25, was killed by police deploying a bomb-carrying robot against him.

Additional reporting by Reuters.