People perform for North Korean leader Kim Jong Un during a celebration for nuclear scientists and engineers who contributed to a hydrogen bomb test, in this undated photo released by North Korea's Korean Central News Agency (KCNA) in Pyongyang. Reuters

SEOUL, South Korea - North Korea's latest nuclear test probably had a yield of 250 kilotons, a US monitoring group said Wednesday -- much higher than official estimates.

Pyongyang conducted its sixth and largest nuclear test last week -- saying it was a hydrogen bomb that could be fitted into a missile -- prompting global condemnation and heightening tensions over its weapons ambitions.

The US Geological Service put the magnitude of the resulting earthquake at 6.3, and the Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO) and Norwegian agency NORSAR had raised their initial figures to 6.1.

As a result, the respected US website 38 North, which is linked to Johns Hopkins University, said it was raising its estimate for the yield of the blast to "roughly 250 kilotons".

The figure is more than 16 times the size of the 15-kiloton US bomb that destroyed Hiroshima in 1945.

"This large explosive yield is also quite close to what 38 North had previously determined to be the maximum estimated containable yield for the Punggye-ri test site," said 38 North.

A man walks past a street monitor showing a news report about North Korea's nuclear test in Tokyo. Reuters

Governmental estimates of the yield vary from South Korea's 50 kilotons to Japan's 160.

US officials have said they are still assessing whether it was an H-bomb, also known as a thermonuclear weapon, but that "so far there is nothing inconsistent with the North Korean claim that this was a hydrogen bomb".

According to 38 North, satellite pictures from last Friday, five days after the test, showed new activity in alternate tunnel portal areas at the Punggye-ri test site including parked trucks, mining carts and other equipments.

"Onsite work could now be changing focus to further prepare those other portals for future underground nuclear testing," it said.

Pyongyang has staged a series of missile tests in recent months that appeared to bring much of the US mainland into range, followed by the September 3 nuclear blast.

It prompted the United Nations Security Council to adopt its eighth set of sanctions on North Korea, but previous resolutions have done little to halt Pyongyang's weapons ambitions.

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