SEOUL (AFP) - North Korea appeared to carry out a sixth nuclear test Sunday (Sept 3), with seismic monitors measuring an "explosion" of 6.3 magnitude near its main test site.

Japan's government said it confirmed a nuclear test had been carried out.

Hours earlier, Pyongyang's state media claimed that the country had developed a thermonuclear warhead that could be fitted onto its new intercontinental ballistic missile, another brazen assertion of its weapons capabilities.

The official Korean Central News Agency said leader Kim Jong Un had inspected a miniaturised H-bomb that could be loaded onto a missile - although doubts remain over the veracity of Pyongyang's claims.

Here are key dates in the North's quest to develop a nuclear-tipped missile capable of hitting the United States:

Late 1970s: North Korea starts working on a version of the Soviet Scud-B (range 300km). Test-fired in 1984.

1987-92: Begins developing variant of Scud-C (range 500km), Rodong-1 (1,300km), Taepodong-1 (2,500km), Musudan-1 (3,000km) and Taepodong-2 (6,700km).

Aug 1998: Test-fires Taepodong-1 rocket over Japan in what it calls a satellite launch - the US and others say it is a missile.

Sept 1999: Declares moratorium on long-range missile tests amid improving ties with US.

July 12, 2000: Fifth round of US-North Korean missile talks ends without agreement after North demands US$1 billion a year in return for halting missile exports.

March 3, 2005: Pyongyang ends moratorium on long-range missile testing, blames Bush administration's "hostile" policy.

July 5, 2006: Test-fires seven missiles, including a long-range Taepodong-2 which explodes after 40 seconds.

Oct 9, 2006: Conducts underground nuclear test, its first.

April 5, 2009: Launches long-range rocket which flies over Japan and lands in the Pacific, in what it says is an attempt to put a satellite into orbit. The United States, Japan and South Korea see it as a disguised test of a Taepodong-2.

May 25, 2009: Conducts its second underground nuclear test, several times more powerful than the first.

April 13, 2012: Launches what it has said is a long-range rocket to put a satellite into orbit, but which disintegrates soon after blast-off.

Dec12, 2012: Launches a multi-stage rocket and successfully places an Earth observational satellite in orbit.

Feb 12, 2013: Conducts its third underground nuclear test.

Jan 6, 2016: Conducts its fourth underground nuclear test, which it says was a hydrogen bomb - a claim doubted by most experts.

March 9, 2016: Kim Jong Un claims the North has successfully miniaturised a thermo-nuclear warhead.

April 23, 2016: Pyongyang test-fires a submarine-launched ballistic missile.

July 8, 2016: US and South Korea announce plans to deploy an advanced missile defence system - Thaad (Terminal High Altitude Area Defence).

Aug 3, 2016: North Korea fires a ballistic missile directly into Japan's maritime economic zone for the first time.

Sept 9, 2016: Conducts fifth nuclear test, its most powerful to date.

March 6, 2017: Fires four ballistic missiles in what it says is an exercise to hit US bases in Japan.

March 7, 2017: US begins deploying Thaad missile defence system in South Korea.

May 14, 2017: North Korea fires a ballistic missile which flies 700 kilometres before landing in the Sea of Japan. Analysts say it has an imputed range of 4,500km and brings Guam within reach.

July 4, 2017: Test-fires a ballistic missile that analysts say brings Alaska within reach. Pyongyang later says it was a "landmark" test of a Hwasong-14 intercontinental ballistic missile (ICBM).

July 28, 2017: Launches a missile with a theoretical range of 10,000km, meaning it could hit much of the United States.

Aug 26, 2017: Fires three short-range ballistic missiles.

Aug 29, 2017: Fires ballistic missile over Japan and into the Pacific, acknowledging for the first time that it has done so. South Korea says it flew around 2,700km at a maximum altitude of about 550km.

Sept 3, 2017: North Korea appears to carry out sixth nuclear test, with seismic monitors measuring an "explosion" of 6.3 magnitude near its main test site. Japan's government confirms a nuclear test has been carried out.

The apparent test comes hours after Pyongyang state media shows leader Kim Jong Un inspecting what it professes to be an H-bomb that can be loaded onto an ICBM. The claims have not been confirmed.