GENEVA (REUTERS) - The top United Nations human rights official called on United States authorities on Tuesday (June 14) to adopt "robust gun control measures" to protect citizens in the wake of the latest mass killing in Orlando, Florida.

"It is hard to find a rational justification that explains the ease with which people can buy firearms, including assault rifles, in spite of prior criminal backgrounds, drug use, histories of domestic violence and mental illness, or direct contact with extremists - both domestic and foreign," UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein said in a statement after a gunman shot dead 49 people in a gay club on Sunday.

On Sunday, 49 people at Orlando nightclub Pulse were shot dead by gunman Omar Mateen, before he was killed by police who stormed the club with armoured cars after a three-hour siege.

US authorities said they had found no direct links between Islamic State in Iraq and Syria (ISIS) and Mateen, but that the gunman had expressed sympathy for a variety of Islamist extremists, including groups in the Middle East that are sworn enemies.