US President Barack Obama said on Sunday that Americans "need to temper our words and open our hearts," in the wake of another spate of shootings targeting police officers.

The nation was still reeling from a mass shooting at a rally in Dallas, which led to the deaths of five police officers, when three more officers, this time in Baton Rouge, were shot dead at a convenience store.

The gunman was later revealed to be a former Marine named Gavin Long, who also wounded three other policemen at the scene, one critically.

Calls for unity

"We as a nation have to be loud and clear that nothing justifies attacks on law enforcement," Obama said during a press conference at the White House on Sunday. "Everyone right now focus on words and actions that can unite this country rather than divide it further."

"We don't need careless accusations thrown around to score political points or to advance an agenda," he added, noting that the country is preparing for the Republican and Democratic National Conventions, a time when emotions tend to run high.

He also emphasized that justice would be achieved for the men who lost their lives.

'An assault on all of us'

Democratic presidential candidate Hillary Clinton was also among those leaders who spoke out following the incident, calling the officers' murders "an assault on all of us."

Republican hopeful Donald Trump, who is set to take the stage at the party's national convention in Cleveland this week, said the country was "divided and out of control" on Twitter.

Baton Rouge was where, earlier this month, a white police officer shot to death a black man, Alton Sterling, after he had been tackled to the ground. The shooting was one of a string of similar incidents that led to massive protests across the country - including the rally in Dallas that ended in bloodshed.

blc/jr (AP, dpa, Reuters, AFP)