(Adds U.S. State Department comment, paragraph 23)
By Josh Smith and Hyonhee Shin
SEOUL, April 26 (Reuters) - North Korean leader Kim Jong Un has pledged to speed up development of his country's nuclear arsenal while overseeing a huge military parade that displayed intercontinental ballistic missiles (ICBMs), state media reported on Tuesday.
The parade took place on Monday night during celebrations for the 90th anniversary of North Korea's armed forces, the state news agency KCNA said.
Pyongyang has recently stepped up weapons tests and displays of military power as denuclearisation talks with the United States have stalled and a new conservative administration takes power in South Korea.
U.S. and South Korean officials say there are signs of new construction at North Korea's only known nuclear test site, which has been officially closed since 2018, suggesting Pyongyang may be preparing to resume testing nuclear weapons.
"The nuclear forces of our Republic should be fully prepared to fulfil their responsible mission and put their unique deterrent in motion at any time," Kim told the gathering, according to KCNA.
He said the fundamental mission of the North's nuclear force was deterrence, but that its use "can never be confined to the single mission".
"If any forces try to violate the fundamental interests of our state, our nuclear forces will have to decisively accomplish an unexpected second mission," Kim said.
Hong Min, a senior fellow at the Korea Institute for National Unification in Seoul, said Kim's speech could signal a change in his nuclear doctrine to leave open the possibility of "nuclear first use".
"Though he did not specify what makes the 'second mission' or 'fundamental interests', he indicated more broadly that the nuclear force might be used pre-emptively, not only when they're under attack, but also under certain circumstances," Hong said.
Yang Moo-jin, a professor at the University of North Korean Studies in Seoul, said Kim's remarks could have been aimed at the incoming government of South Korean president-elect Yoon Suk-yeol, who has warned of possible pre-emptive strikes if an attack from the North is imminent.
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Yoon takes office on May 10. His transition team criticised Pyongyang for developing offensive weapons while appearing to pursue talks.
"The parade proved that North Korea has outwardly called for peace and dialogue over the last five years but in reality it focused on developing the means to threaten not only the Korean peninsula but Northeast Asia and world peace," deputy spokesperson Won Il-hee told a briefing.