WASHINGTON (Reuters) - South Korean President Moon Jae In said he hopes to draw North Korea to the negotiating table over its nuclear programme by the end of the year and pushed off any talk about a potential preemptive strike, according to a US media interview that aired on Tuesday (June 20).

Asked about the possibility of such a strike, Moon told CBS News in a televised interview: "When it comes to pre-emptive strike ... this is something we may be able to discuss at a later date when the threat has become even more urgent."

He also said North Korea should swiftly return South Koreans and Americans detained in the reclusive nation and that it had "a heavy responsibility" in the death of a US university student.

Otto Warmbier, the 22-year-old student 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, his family said.

Three other US citizens, who are ethnic Koreans, and six South Koreans remain in custody in North Korea.

Warmbier was arrested while visiting as a tourist and accused of trying to steal an item bearing a propaganda slogan, according to North Korean media. Doctors caring for him last week described him as having extensive brain damage that left him in a state of "unresponsive wakefulness."

In the CBS interview, Moon said that while "we cannot know for sure that North Korea killed Mr Warmbier ... I believe it is quite clear that they have a heavy responsibility in the process that led to Mr Warmbier's death."

"I believe we must now have the perception that North Korea is an irrational regime," Moon told the CBS television network.

South Korea's Blue House on Tuesday cited Moon separately as saying: "It is very deplorable that North Korea does not respect human rights."

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.

US President Donald Trump blamed the "brutality of the North Korean regime" for Warmbier's death.

North Korea said last month it was its sovereign right to"ruthlessly punish" US citizens it had detained for crimes against the state.

Korean-Americans Tony Kim and Kim Hak Song, who worked at the foreign-funded Pyongyang University of Science and Technology, were recently detained for hostile acts against the government, according to North Korea's state media.

In March 2016, Kim Dong Chul, a 62-year-old Korean-American missionary, was sentenced to 10 years of hard labor for subversion.

North Korea is also holding Canadian pastor Hyeon Soo Lim. He was charged with subversion and given a hard-labour life sentence in 2015.

Three South Korean nationals were detained in North Korea during their missionary work since 2013, and the remaining three South Koreans are North Korean defectors who returned and are in custody, a lawmaker briefed by the South Korean spy agency told reporters last week.

Dozens of North Korean missile launches and two nuclear bomb tests since the beginning of last year have heightened tensions on the Korean peninsula. Pyongyang has vowed to develop a nuclear-tipped intercontinental ballistic missile capable of hitting the US mainland.

Pyongyang continued to test-fire missiles since South Korean leader Moon took office pledging to engage in dialogue with North Korea.