Warsaw - US President Barack Obama on Saturday sought to calm fears of a divided nation after five police officers were killed in Dallas during a protest against the shooting of African Americans."America is not as divided as some have suggested," Obama told a press conference at a Nato summit in Warsaw. "There is sorrow, there is anger, there is confusion... but there is unity."With the United States reeling from both a long-running racial row and the latest in a series of mass shootings, Obama denied the country was heading back to the civil unrest of the 1960s."When we start suggesting that somehow there's this enormous polarisation and we're back to the situation in the 60s and - that's just not true," Obama said."You're not seeing riots, you're not seeing police going after people who are protesting peacefully."Obama said Dallas shooter Micah Johnson, 25, who was killed in a stand-off with police, was "demented" and did not represent African Americans."The demented individual who carried out the attacks in Dallas, he's no more representative of African Americans than the shooter in Charleston was representative of white Americans," Obama said.Dylann Roof, 22, is accused of gunning down nine African American churchgoers in South Carolina last year and faces two death penalty trials.