GENEVA (Reuters) - The top United Nations human rights official called on U.S. authorities on Tuesday 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," U.N. 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.

(Reporting by Stephanie Nebehay)