North Korean leader Kim Jong Un has pledged to continue the country's policy of simultaneously developing the economy and nuclear weapons, official media said Sunday, amid a tense diplomatic standoff with the United States.

Kim made the pledge on Saturday in a plenary meeting of the Central Committee of the Workers' Party of Korea, held for the first time since May last year, which focused on two agenda items: the situation around North Korea and a reshuffle of senior members.

His younger sister Kim Yo Jong, who often accompanies him on official duties, was promoted to become an alternate member of the committee's Political Bureau, in another sign that she is gaining a stronger foothold in the country's opaque power structure.

The sister, a vice director of the party's Propaganda and Agitation Department who is believed to be around 30 years old, was elected to the Central Committee only in May last year when the party held its first congress in 36 years.

During the plenum, Kim, chairman of the ruling party, accused the United States of trying to "completely stifle the sovereignty, the rights to existence and development" of his country by "cooking up" U.N. sanctions resolutions "one after another with mobilization of (its) vassal forces," according to the Korean Central News Agency.

Kim said the current situation has proved that the party was "absolutely right" to uphold the simultaneous development policy and that it should "invariably keep to this road in the future."

He was quoted as saying that North Korea's nuclear arms are "a precious fruition borne by its people's bloody struggle for defending the destiny and sovereignty of the country from the protracted nuclear threats of the U.S. imperialists."

The plenary meeting was held a day before the 20th anniversary of Kim Jong Il, the current leader's father, being elected as general secretary of the party.

North Korea organized a major celebration for the anniversary of the late leader this year.

Choe Ryong Hae, a vice chairman of the party, delivered a speech at a gathering attended by 100,000 people from all walks of life on Kim Il Sung Square in the heart of Pyongyang.

Choe praised Kim Jong Il's contributions to strengthening North Korea's national defense and the guidance of the current leader who has turned the country into a military power with the possession of atomic and hydrogen bombs, and intercontinental ballistic missiles.

The capital city's major locations were adorned with national flags and signs commemorating the anniversary. A mass dance on the square involving university students and a fireworks display were set to be held in the evening.

North Korea will also mark the 72nd anniversary of the ruling party's founding on Tuesday, and there has been speculation abroad that it may conduct another major weapons test on or around that day.

U.S. President Donald Trump on Saturday again suggested he does not rule out taking military action, if necessary, to deal with North Korea after previous administrations' efforts to curb its nuclear and missile programs through dialogue failed.

"Presidents and their administrations have been talking to North Korea for 25 years, agreements made and massive amounts of money paid...hasn't worked, agreements violated before the ink was dry, making fools of U.S. negotiators," Trump said in a pair of Twitter posts.

The U.S. president continued, without elaborating, "Sorry, but only one thing will work!"

Among other personnel changes disclosed after the plenary meeting, Choe, a very close aide to the North Korean leader, became a member of the party's powerful Central Military Commission and Foreign Minister Ri Yong Ho was elected as a member of the Political Bureau.

Six senior members, including An Jong Su, light industry minister, and Choe Hwi, who deals with propaganda affairs in the same department as Kim's sister, were promoted to become vice chairmen of the committee.

The KCNA report, however, has no mention of who left high-ranking positions in the reshuffle.

The ruling party must hold a plenary meeting at least once a year under its rules.