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, Reuters reported.

Choe Son Hui, long a key member of Pyongyang's team negotiating over its nuclear program 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-defense 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 at the Plenary Meeting of the Central Committee of the Workers' Party of Korea.