Obama condemns Dallas attacks

US President Barack Obama has denounced the despicable attacks in Dallas in which snipers shot dead five police officers during a protest against police shootings of black men.

The snipers operating from rooftops in Dallas killed the five officers and wounded seven others along with two civilians in the attack during one of several protests across the US against the killing of Philando Castile in Minnesota and Alton Sterling in Lousiana earlier this week.

Following his arrival in Warsaw to attend NATO summit, Obama called on the American people to exert greater efforts to address the violent relationship between the local police and American blacks and Latinos, after the shooting of two African American men by police in the states of Minnesota and Louisiana in two days.

The US president extended his condolences by telephone to Mike Rawlings, Mayor of Dallas and also talked to the family of a black young man who was killed in Minnesota.

Police described last night's ambush as carefully planned and executed and had taken three people into custody. A fourth died from what Dallas-based media said was a self-inflicted gunshot after a standoff that extended into this morning.

That suspect had exchanged gunfire with police during the stand-off at a downtown garage and warned of placing bombs throughout the city. A police spokesman later said no explosives have been found after "primary and secondary sweeps" of the downtown area.

The attack was one of the worst mass shootings of police in US history.