Pyongyang's declaration, long sought by Washington, will be seen as a crucial step in the fast diplomatic dance on and around the Korean peninsula.

It comes less than a week before the North Korean leader meets South Korean President Moon Jae-in for a summit in the Demilitarised Zone that divides the peninsula, ahead of a much-anticipated encounter with Trump himself.

The North had developed its weapons in what he called a "great victory", and so "no nuclear test and intermediate-range and inter-continental ballistic rocket test-fire are necessary for the DPRK now".

"The mission of the northern nuclear test ground has thus come to an end," he added at the gathering of the central committee of the Workers' Party, according to the official KCNA news agency.

The party decided that nuclear blasts and ICBM launches will cease as of Saturday -- the North has not carried any out since November -- and the atomic test site at Punggye-ri will be dismantled to "transparently guarantee" the end of testing.

Within minutes of the report being issued, Trump tweeted: "This is very good news for North Korea and the World - big progress! Look forward to our Summit."

Seoul too welcomed the announcement, calling it "meaningful progress" towards the denuclearisation of the Korean peninsula.

"It will create a very positive environment for the success of the upcoming inter-Korean and North-US summits," the South Korean presidential office said in a statement.

Pyongyang has made rapid technological progress in its weapons programmes under Kim, which has seen it subjected to increasingly strict sanctions by the UN Security Council, the United States, the European Union, South Korea and others.

Last year it carried out its sixth nuclear blast, its most powerful to date, and tested missiles it said are capable of reaching the US mainland.

Kim and Trump traded threats of war and personal insults as tensions ramped up, and even when there was an extended pause in testing, US officials said that it could not be interpreted as a halt without an explicit statement from Pyongyang.

South Korean envoys have previously cited Kim as promising no more tests, but Saturday's news is the first such announcement directly by Pyongyang.

"Certainly this is a positive development," said Daniel Pinkston of Troy University. "It's a necessary but not sufficient step in North Korea returning to its past non-proliferation commitments."

- 'Fresh climate' -

The formal declaration of an end to testing comes after Kim stated in his New Year speech that the development of North Korea's nuclear force had been completed.

In the same address, he said he had a nuclear button on his desk, prompting Trump to tweet that he had a bigger one of his own.

Events have moved rapidly since then, catalysed by the Winter Olympics in the South, and Seoul is now pushing for a peace treaty to formally end the Korean War, raising hopes that a settlement can finally be reached on the peninsula.

But there is a long way to go and Moon himself acknowledged this week that the "devil is in the details".

The US is seeking the complete, verifiable and irreversible denuclearisation of the North, while according to Moon, Pyongyang wants security guarantees, leaving much room for negotiation.

The North has long demanded the withdrawal of US troops from the peninsula and an end to its nuclear umbrella over South Korea, something unthinkable in Washington.

For its part, Pyongyang has been relatively reticent about the process, making its first official reference to contacts with the US only last week, when KCNA said Kim had discussed "the prospect of the DPRK-US dialogue".

But the leader told the Workers' Party meeting: "A fresh climate of detente and peace is being created on the Korean peninsula and the region and dramatic changes are being made in the international political landscape."

- 'New strategic line' -

For years, the impoverished North has pursued a "byungjin" policy of "simultaneous development" of both the military and the economy.

But Kim said that as it was now a powerful state, "the whole party and country" should concentrate on "socialist economic construction", in what he called the party's "new strategic line".

Several factors have driven the Korean rapprochement, including the North feeling that it can now negotiate from a position of strength, concern about the belligerence of the Trump administration, and the looming impact of sanctions.

Professor Yang Moo-jin of the University of North Korean Studies told AFP: "This is in effect a declaration of abandoning the byungjin policy for the sake of economic development."

"The North will now engage in negotiations with the US to obtain the normalisation of ties, guarantee against attempts to undermine its regime, and the lifting of sanctions which are essential for developing economy and improving people's livelihoods," Yang said.