North Korea's 'biggest' nuclear test sparks global outrage
World leaders have reacted with anger after North Korea carried out its fifth and reportedly biggest nuclear test.
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The South accused the North's leader Kim Jong-un of "maniacal recklessness".
China "firmly opposed" the test, Japan "protested adamantly" and the US warned of "serious consequences".
Such tests are banned by the UN, but this is the second nuclear test this year, and Kim Jong-un's rhetoric has become increasingly aggressive.
The isolated communist nation has been hit by five sets of UN sanctions since its first test in 2006. Talks involving world and regional powers have failed to rein in the North's nuclear programme.
In its statement announcing the underground test, North Korea expressed anger at the "racket of threat and sanctions... kicked up by the US-led hostile forces" to deny a "sovereign state's exercise of the right to self-defence".
The test came on the country's National Day, which celebrates the founding of the current regime and which is often used as a show of military strength.
Technically, the North said the test was aimed at further developing the miniaturisation of nuclear warheads so they could be mounted on ballistic missiles.
In its statement the North said it could now produce "at will, and as many as it wants, a variety of smaller, lighter and diversified nuclear warheads of higher strike power".
In recent months, the North has conducted a series of ballistic missile launches and has in the past often stated its aim of hitting US targets.
The North has previously made claims on "miniaturised" nuclear warheads but they have never been independently confirmed.
North Korea has also been angered by a US and South Korean plan to install an anti-missile defence system in the South and by the allies' massive annual joint military exercises, which are still taking place.
The North's recent actions have sorely tested its only ally, China.
It condemned January's test and repeated that on Friday after the latest.
China's foreign ministry said it would lodge a diplomatic protest and urged North Korea to avoid further action that would worsen the situation.
The latest test was announced on state TV hours after a 5.3 magnitude tremor was detected near the Punggye-ri underground nuclear site.
Estimates of the explosive yield of the latest blast have varied. South Korea's military said it was about 10 kilotonnes, enough to make it the North's "strongest nuclear test ever". Other experts say initial indications suggest 20 kilotonnes or more.
The bomb dropped by the US on Hiroshima in 1945 had a yield of about 15 kilotonnes.