The nuclear bomb reportedly tested by North Korea was around five times larger than the bomb dropped on Nagasaki during World War II.
It is also believed that the weapon was designed to fit atop an intercontinental ballistic missile, a device that would be capable of reaching the mainland US.
North Korean state media reported that the test was a 'complete success' and the final step in attaining a 'state nuclear force'.
Experts have estimated that the explosion was around 10 kilotons, Sky News reports.
The blast caused tremors across the country. NORSAR, an independent seismic monitor, has estimated the blast created a yield of about 120 kilotons.
It comes after Pyongyang conducted two intercontinental ballistic missile tests in July that potentially could fly around 10,000 km (6,200 miles).
Japan's foreign minister, Taro Kono, described the new explosion as 'extremely unforgivable'.
Despite being North Korea's most important international ally, a Chinese foreign ministry statement said North Korea 'has ignored the international community's widespread opposition, again carrying out a nuclear test. China's government expresses resolute opposition and strong condemnation toward this'.
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