Donald Trump appears to have given up on diplomacy efforts with North Korea.

North Korean leader Kim Jong Un says his nuclear weapons are a "powerful deterrent" which guarantee North Korea's sovereignty, hours after US 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 US imperialists."

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

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

Donald Trump has previously said the US 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 said in the speech.

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

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, the promotion makes the 28-year-old the only other millennial member of the influential body.

Her promotion indicates Kim Yo Jong has become a replacement for Kim'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.