North Korean leader Kim Jong Un said his nuclear weapons are a "powerful deterrent" which guarantee North Korea's sovereignty, state media reported on Sunday, hours after U.S. President Donald Trump said "only one thing will work" in dealing with the isolated country.

Trump did not make clear to what he was referring, but his comments seemed to be a further suggestion that military action was on his mind.

In a speech to a meeting of the powerful Central Committee of the ruling Workers' Party on Saturday, a day before Trump's most recent comments, state media said Kim had addressed the "complicated international situation".

North Korea's nuclear weapons are a "powerful deterrent firmly safeguarding the peace and security in the Korean peninsula and Northeast Asia," Kim said, referring to the "protracted nuclear threats of the U.S. imperialists."

In recent weeks, North Korea has launched two missiles over Japan and conducted its sixth nuclear test, and may be fast advancing towards its goal of developing a nuclear-tipped missile capable of hitting the U.S. mainland.

North Korea is preparing to test-launch such a missile, a Russian lawmaker who had just returned from a visit to Pyongyang was quoted as saying on Friday.

Donald Trump has previously said the United States would "totally destroy" North Korea if necessary to protect itself and its allies.

The situation proved that North Korea's policy of "byungjin", meaning the parallel development of nuclear weapons and the economy was "absolutely right", Kim Jong Un said in the speech.

"The national economy has grown on their strength this year, despite the escalating sanctions," said Kim, referring to U.N. Security Council resolutions put in place to curb Pyongyang's nuclear and missile programmes.

SISTER PROMOTION

The meeting also handled some personnel changes inside North Korea's secretive and opaque ruling centre of power, state media said.

Kim Jong Un's sister, Kim Yo Jong, was made an alternate member of the politburo - the top decision-making body over which Kim Jong Un presides.

Alongside Kim Jong Un himself, the promotion makes the 30-year-old the only other millennial member of the influential body.

Her promotion indicates the 28-year-old has become a replacement for Kim Jong Un's aunt, Kim Kyong Hee, who had been a key decision maker when former leader Kim Jong Il was alive.

"It shows that her portfolio and writ is far more substantive than previously believed and it is a further consolidation of the Kim family's power," said Michael Madden, a North Korea expert at Johns Hopkins University's 38 North website.

In January, the U.S. Treasury blacklisted Kim Yo Jong along with other North Korean officials over "severe human rights abuses".

Kim Jong Sik and Ri Pyong Chol, two of the three men behind Kim's banned rocket programme, were also promoted.

North Korea's foreign minister Ri Yong Ho, who named Donald Trump "President Evil" in a bombastic speech to the U.N. General Assembly last month, was promoted to full vote-carrying member of the politburo.

"Ri can now be safely identified as one of North Korea's top policy makers," said Madden.

"Even if he has informal or off the record meetings, Ri's interlocutors can be assured that whatever proposals they proffer will be taken directly to the top," he said.