Prime Minister Lee Nak-yon on Sunday urged North Korea to free South Koreans and Americans detained in the reclusive country in the wake of the death of an American college student who died days after being release in a coma.

During a ceremony marking the 67th anniversary of the Korean War, Lee said Pyongyang should stop developing its nuclear and missile capabilities. The two countries are technically still at war after the 1950-53 war ended in a truce, not a peace treaty.

"The international community is outraged about the latest death of American Warmbier," Lee said. "North Korea should come forward for denuclearization of the Korean Peninsula and respect human rights."

Otto Warmbier, 22, died after he was flown to the United States following 17 months of detention for taking down a political propaganda sign from a hotel during a trip.

There are three more Americans held in North Korea. Kim Dong-chul, a 62-year-old Korean-American, was arrested in 2015 and sentenced to 10 years of hard labor on charges of espionage and subversion.

The two others, Kim Hak-song and Kim Sang-dok, were taken into custody earlier this year. They worked at the Pyongyang University of Science and Technology in North Korea.

Pyongyang is also holding six South Koreans including three missionaries and one pastor.

According to Japanese new media, the North is known to have asked the US to send one of its former presidents as an envoy for a negotiation to release the detainees.

The US, however, sent Joseph Yun, US Special Representative for North Korea Policy, and brought Warmbier back home. (Yonhap)