"The worldwide community must treat this new provocation with the utmost firmness, in order to bring North Korea to come back unconditionally to the path of dialogue and to proceed to the complete, verifiable and irreversible dismantling of its nuclear and ballistic programme", he said in a statement.
China, under fire from Trump for failing to restrain its neighbour and ally, has pushed for a suspension of the North's nuclear and missile tests in exchange for a halt to the drills - a quid pro quo firmly rejected by Seoul and Washington. Seoul officials revised their earlier estimate of 5.6 magnitude quake.
Initial reports from the US Geological Survey put the tremor at 5.6 magnitude with a depth of 10km (six miles) but this was later upgraded to 6.3 magnitude at 0km.
The U.S. State Department had no immediate reaction.
South Korea on Sunday called for the "strongest possible" response, including new U.N. Security Council sanctions to "completely isolate" North Korea.
"The government confirms that North Korea conducted a nuclear test after examining information from the weather agency and other information", said Taro Kono, Japan's foreign minister.
The explosion was heralded by a 6.3-magnitude natural disaster felt in Yanji, China, about six miles (10km) from North Korea's Punggye-ri nuclear test site in the north-east of the country.
"The worldwide community must treat this new provocation with the utmost firmness, in order to bring North Korea to come back unconditionally to the path of dialogue and to proceed to the complete, verifiable and irreversible dismantling of its nuclear and ballistic programme". It has since maintained a torrid pace in weapons tests, including its first two intercontinental ballistic missiles test in July. It's also a reminder of the challenge that North Korea presents for the Trump administration.
The explosive power of the bomb is adjustable from tens of kilotons to hundreds of kilotons, KCNA said.
The country has defied United Nations sanctions and worldwide pressure to develop nuclear weapons and to test missiles which could potentially reach the mainland US.
The North followed its two tests of Hwasong-14 ICBMs by announcing a plan to launch a salvo of intermediate range missiles toward the U.S. Pacific island territory of Guam.
Vipin Narang, a Massachusetts Institute of Technology professor specializing in nuclear strategy, said before the natural disaster that it's important to note that the photos showed only a mockup of a two-stage thermonuclear device, or H-bomb. In a statement, the ministry said on its website that China "resolutely opposed" and "strongly condemned" North Korea's actions.
The test re-confirmed the precision of the compression technology of the first system of the H-bomb and the fission chain reaction start control technology and proved once again that the nuclear material utility rate in the first system and the second system reached the levels reflected in the design.
China, North Korea's only major ally, said North Korea "has ignored the global community's widespread opposition, again carrying out a nuclear test".
It is nearly impossible to independently confirm North Korean statements about its highly secret weapons program.
Lassina Zerbo, the executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organisation, said that more than 34 stations picked up the seismic activity from the blast.
North Korea last carried out a nuclear test in September 2016.
Sept 1999: Declares moratorium on long-range missile tests amid improving ties with US.
July 8, 2016: US and South Korea announce plans to deploy an advanced missile defence system - THAAD (Terminal High Altitude Area Defense).
North Korea's sixth nuclear test - probably its largest so far - sends out one clear political signal.
An announcement from KCNA, Pyongyang's state news agency, said the H-bomb, created to be the payload of an ICBM, was "true to the Workers' Party of Korea's plan for building a strategic nuclear force".
Huang Tao, who works at a state-owned company in Yanji, found out about the nuclear test when his friends posted it on WeChat, and noted that Sunday is marked in China as the 72nd anniversary of Japan's surrender in World War Two.
March 7, 2017: United States begins deploying THAAD missile defence system in South Korea.