Up to 600 North Koreans have "vanished" after being forcibly deported by China in October, a Seoul-based human rights group said on Thursday, warning they may face imprisonment, torture, sexual violence and execution in the isolated state.

The report by the Transitional Justice Working Group (TJWG) came about two months after South Korea lodged a protest with China over the suspected repatriation of a large of number of North Koreans who were trying to flee to South Korea.

The TJWG said hundreds of defectors were transported in guarded buses and vans from Chinese detention centers across the border into the North on Oct. 9, calling the incident the largest such mass repatriation in years.