SEOUL, June 11 (Reuters) - North Korea appointed a key nuclear negotiator, Choe Son Hui, as its new foreign minister, state media said on Saturday, as the country concluded a ruling party meeting chaired by leader Kim Jong Un.

Kim presented goals to boost the country's military power and defence research to protect North Korea's sovereign rights, state news agency KCNA said, but it did not give details.

"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 by the KCNA.

The Plenary Meeting of the Central Committee of the Workers' Party of Korea (WPK) was convened on Wednesday through Friday, KCNA said.

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

On Wednesday, U.S. national security adviser Jake Sullivan said the United States is watching "very closely" the continued possibility of a nuclear test by North Korea.

South Korea's defense minister Lee Jong-sup and U.S. Defense Secretary Lloyd Austin are expected to meet on Saturday at Shangri-La Dialogue in Singapore.