The United States has extended its travel restriction for North Korea by another year, the State Department said Wednesday, citing a "serious" risk to its citizens of arrest and long-term detention in the reclusive state.The department posted on the Federal Register the decision to extend the restriction until Aug. 31 next year. Since it was imposed in September 2017, the restriction has been extended annually."The Department of State has determined there continues to be serious risk to U.S. citizens and nationals of arrest and long-term detention constituting imminent danger to their physical safety," the department said in a notice on the register."Accordingly, all U.S. passports shall remain invalid for travel to, in, or through the DPRK unless specially validated for such travel under the authority of the Secretary of State," it added, referring to the North by its official name, the Democratic People's Republic of Korea.The restriction was introduced after Otto Warmbier, an American college stud ent, was released from the North in a vegetative state and died in 2017. He was imprisoned in the North on a charge of subversion the previous year.Source : Yonhap News Agency