SEOUL: North Korea declared itself a thermonuclear power on Sunday, after carrying out a sixth nuclear test more powerful than any it has previously detonated, presenting President Donald Trump with a potent challenge.

The North has tested a hydrogen bomb with "perfect success", a jubilant newsreader announced on state television, adding the device could be mounted on a missile.

The test was of a bomb with "unprecedently large power", she said, and "marked a very significant occasion in attaining the final goal of completing the state nuclear force".

The broadcaster showed an image of leader Kim Jong-Un's handwritten order for the test to be carried out at noon on September 3.

The announcement came after monitors measured a 6.3-magnitude tremor near the North's main testing site, which South Korean experts said was five to six times stronger than that from the 10-kiloton test carried out a year ago.

Hours earlier, the North released images of Kim inspecting what it said was a miniaturised H-bomb that could be fitted onto an ICBM, at the Nuclear Weapons Institute.

Hydrogen bombs or H-bombs -- also known as thermonuclear devices -- are far more powerful than the relatively simple atomic weapons the North was believed to have tested so far.

Whatever the final figure for test's yield turned out to be, said Jeffrey Lewis of the armscontrolwonk website, it was "a staged thermonuclear weapon" which represents a significant advance in its weapons program.

Chinese monitors said they had detected a second quake shortly afterwards of 4.6 magnitude that could be due to a "collapse (cave in)", suggesting the rock over the underground blast had given way.

Pyongyang has long sought the means to deliver an atomic warhead to the United States, its sworn enemy, and the test will infuriate Washington, Tokyo, Seoul, Beijing and others.

Japanese Prime Minister Shinzo Abe said ahead of the announcement that a test would be "absolutely unacceptable".

South Korean President Moon Jae-In summoned the National Security Council for an emergency meeting and Seoul's military raised its alert level.

'Super explosive power'

Pyongyang triggered a new ramping up of tensions in July, when it carried out two successful tests of an ICBM, the Hwasong-14, which apparently brought much of the US mainland within range.

It has since threatened to send a salvo of rockets towards the US territory of Guam, and last week fired a missile over Japan and into the Pacific, the first time time it has ever acknowledged doing so.

Trump has warned Pyongyang that it faces "fire and fury", and that Washington's weapons are "locked and loaded".

Analysts believe Pyongyang has been developing weapons capability to give it a stronger hand in any negotiations with the US.

"North Korea will continue with their nuclear weapons programme unless the US proposes talks,"  Koo Kab-Woo of Seoul's University of North Korean Studies told AFP.

He pointed to the fact that Pakistan -- whose nuclear programme is believed to have links with the North's -- conducted six nuclear tests in total, and may not have seen a need for any further blasts.

"If we look at it from Pakistan's example, the North might be in the final stages" of becoming a nuclear state, he said.

Pictures of Kim at the Nuclear Weapons Institute showed the young leader, dressed in a black suit, examining a metal casing with a shape akin to a peanut shell.

The device was a "thermonuclear weapon with super explosive power made by our own efforts and technology", KCNA cited Kim as saying, and "all components of the H-bomb were 100 percent domestically made".

Actually mounting a warhead onto a missile would amount to a significant escalation on the North's part, as it would create a risk that it was preparing an attack.

Failure of sanctions

The North carried out its first nuclear test in 2006, and successive blasts are believed to have been aimed at refining designs and reliability as well as increasing yield.

Its fifth detonation, in September last year, caused a 5.3 magnitude quake and according to Seoul had a 10-kiloton yield -- still less than the 15-kiloton US device which destroyed Hiroshima in 1945.

The North Korean leadership says a credible nuclear deterrent is critical to the nation's survival, claiming it is under constant threat from an aggressive United States.

It has been subjected to seven rounds of United Nations Security Council sanctions over its nuclear and ballistic missile programmes, but always insists it will continue to pursue them.

Atomic or "A-bombs" work on the principle of nuclear fission, where energy is released by splitting atoms of enriched uranium or plutonium encased in the warhead.