Veteran former U.S. politician and diplomat Bill Richardson offered on Friday to visit North Korea to secure the release of three detained Americans and a Canadian after U.S. university student Otto Warmbier was brought back in a coma.

Richardson, a former New Mexico governor and congressman who visited North Korea in the 1990s and sought the release of detained Americans, said he had made the proposal in a letter to the North Korean mission to the United Nations.

"I have proposed that as of today," Richardson said, adding that he argued in the letter it is in North Korea's interest to free the detainees unconditionally, "in the light of its failure to properly take care and handle" Warmbier's case.