Warsaw: US President Barack Obama on Friday condemned the shooting of five police officials when snipers opened fire during a protest over the recent fatal shootings of two black men by white police in the Minnesota and Louisiana this week. The shooting occurred during a protest against police violence that Obama had also decried, Efe news reported. "This was a vicious, calculated and despicable attack," Obama said of the Dallas shootings at a press conference during the North Atlantic Treaty Organization Summit in Warsaw. Obama vowed that anyone involved in the senseless murders would be held fully accountable. "When people say 'black lives matter', it doesn't mean that blue lives don't matter," Obama said. He also said data showed black citizens were more vulnerable to acts of violence than their white neighbours. "There is no possible justification for these kinds of attacks or any violence against law enforcement," he said, emphasising that "justice will be done". Obama said he spoke over phone with Dallas mayor Mike Rawlings and offered any federal assistance the city may need. The US has lived several moments of racial tension in the past two years, especially following the death in 2014 of Michael Brown, who was killed by a police officer despite being unarmed and not a criminal. Protesters returned to the streets this week following the deaths of Alto Sterling and Philando Castile at the hands of police in Louisiana and Minnesota.