A defiant North Korea has restated its demand for recognition as a "legitimate" nuclear-armed state, as world powers ponder ways to punish Pyongyang for its latest and largest atomic test.

Key points: North Korea vows to increase nuclear force "in quality and quantity"

North Korea vows to increase nuclear force "in quality and quantity" Washington, Tokyo seek "strongest possible" measures, US envoy says

Washington, Tokyo seek "strongest possible" measures, US envoy says US vows to defend Seoul using "all means possible"

North Korea also vowed to increase its nuclear strike force "in quality and in quantity", two days after its fifth test in a decade sparked international condemnation and moves for tougher United Nations sanctions.

In Japan, a visiting senior US envoy said Washington and Tokyo were seeking "the strongest possible" measures in response.

North Korea insists its missile and nuclear tests are necessary to counter what it says is a US nuclear threat to its independence.

A statement on Sunday from a foreign ministry spokesman in Pyongyang mocked President Barack Obama's "totally bankrupt" policy on the country.

"Obama is trying hard to deny the DPRK's (North Korea's) strategic position as a legitimate nuclear weapons state, but it is as foolish an act as trying to eclipse the sun with a palm," the statement quoted by the official KCNA news agency said.

Friday's test came only eight months after the previous one and was almost twice as powerful, at an estimated 10 kilotons.

Most worryingly for the international community, the North claimed it had been a miniaturised warhead that could be mounted on a missile.

The UN Security Council agreed on Friday to start work on new measures, even though five sets of UN sanctions since the first nuclear test in 2006 have failed to halt the North's nuclear drive.

Sung Kim, the US State Department's special representative for North Korea policy, suggested the US may launch its own unilateral sanctions in response to "the provocative and unacceptable behaviour by the North Koreans".

'Test demonstrated dignity'

KCNA insisted on Sunday that North Koreans were delighted by Friday's detonation.

"The test demonstrated the dignity of the country guided by the iron-willed commander as well as the strength of the Korean people," scientist Choe Kwang-Ho was quoted as saying.

A television screen in Seoul shows Kim Jong-Un celebrating the missile test in August. ( AFP: Jung Yeon-Je )

The Government in Seoul will take "all diplomatic and military efforts to counter North Korea's continued provocation", senior presidential secretary for foreign affairs Kim Kyou-Hyun said.

Referring to Friday's phone conversation between President Park Geun-Hye and Mr Obama, he said the United States had vowed to defend Seoul using "all means available" — including the nuclear umbrella and conventional forces.

The South's military has said it would launch a retaliatory strike at Pyongyang's military leadership if it deemed the country was under nuclear threat.

Dubbed the Korea Massive Punishment and Retaliation system, the countermeasure would "directly target" key North Korean sites — including its war command post — if any sign of a nuclear attack was detected.

AFP