North Korea released a South Korean citizen detained in July after crossing the border into the North, the South's Unification Ministry said on Tuesday.

The prisoner was a 34-year-old man surnamed Seo who was detained after "illegally" entering North Korea last month, the ministry said.

South Korean officials have pressed for the return of six other South Koreans believed to have been held for years in North Korea, and the Unification Ministry said they believed Seo's release is a positive sign.

After the Americans were released, the United Nations human rights investigator on North Korea, Tomas Ojea Quintana, issued a statement calling for the South Koreans to also be freed.

Among them are Christian missionaries Kim Jung-wook, who was arrested in October 2013, and Kim Kook-kie and Choi Chun-kil, who have been held since 2014, the ministry said.