Houston: US President Barack Obama today asked Americans to "reject despair" and insisted that the country is not as divided as it seems by race and politics as he paid tributes to five police officers killed during a deadly sniper attack in Dallas.

"We see all this and it's hard not to think, sometimes, that the center won't hold and that things could get worse," he said. "I understand. I understand how Americans are feeling. But Dallas, I'm here to say we must reject such despair. I'm here to insist that we are not as divided as we seem, and I know that because I know America. I know how far we've come against impossible odds."