North Korea's Kim Jong Un vows 'power to power' military; promotes nuclear envoy

North Korea promoted its key nuclear negotiator to foreign minister, state media said on Saturday, as leader Kim Jong Un vowed to his ruling party that he would use "power for power" to fight threats to the country's sovereignty.

Choe Son Hui, long a key member of Pyongyang's team negotiating over its nuclear programme with the United States, was named foreign minister, state news agency KCNA said.

The appointment comes as the United States warned this month that North Korea is preparing to conduct a seventh nuclear test, and says it will again push for United Nations sanctions if that happens.

Kim did not mention a nuclear test and offered no details about how he would bolster military power as international concerns grow that he would order the first such test in five years.

"The right to self-defence is an issue of defending sovereignty, clarifying once again the party's invariable fighting principle of power for power and head-on contest," Kim was quoted as saying.

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He announced goals to boost the country's military power and defence research to protect North Korea's sovereign rights, KCNA said in a report of a Plenary Meeting of the Central Committee of the Workers' Party of Korea.

South Korean Defence Minister Lee Jong-sup and US Defense Secretary Lloyd Austin on Saturday condemned the North's ongoing military activities and preparations for a nuclear test as unacceptable provocations.

Meeting on the sidelines of a security conference in Singapore, Lee and Austin, "agreed to expand the scope and scale of South Korea-US combined exercises as agreed at the two countries' summit in order to maintain steadfast deterrence and constant readiness," South Korea's Defence Ministry said in a statement.

"Secretary Austin underscored that the US commitment to the defence of (South Korea) is ironclad and underpinned by the full range of US capabilities, including nuclear," the US Defense Department said in a statement.

US National Security Adviser Jake Sullivan said on Wednesday that Washington is watching "very closely" the continued possibility of a nuclear test by North Korea.