South Korean President Moon Jae-in delivers a speech in Seoul, South Korea, June 6, 2017. (Photo: REUTERS/Kim Hong-Ji)

The South Korean government will make every effort for the return of those held in North Korea, presidential spokesman Park Soo-hyun told a briefing on Tuesday. American university student Otto Warmbier, who had been held prisoner in North Korea for 17 months, died at a Cincinnati hospital on Monday just days after North Korea released him from captivity in a coma. US Secretary of State Rex Tillerson said the US holds North Korea "accountable" for the imprisonment. Three other US citizens, who are ethnic Koreans, and six South Koreans remain in custody in North Korea.

The same day, two US long-range strategic bombers trained with South Korea's fighter jets over the Korean peninsula as part of a regular combined exercise, South Korean defense officials said. The sortie of the US bombers based in Guam came after the death of Warmbier.