South Korea has said that North Korea's nuclear capability is expanding fast, echoing alarm around the world over the isolated state's fifth and biggest nuclear test, carried out in defiance of UN sanctions.

North Korea conducted the test yesterday and said it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that rivals and the United Nations have been powerless to contain.

The test showed North Korean leader Kim Jong Un was unwilling to alter course, and that tougher sanctions and pressure were needed to apply "unbearable pain on the North to leave no choice but to change", South Korean Foreign Minister Yun Byung-se said.

"North Korea's nuclear capability is growing and speeding to a considerable level, considering the fifth nuclear test was the strongest in scale and the interval has quickened substantially," Mr Yun told a ministry meeting convened to discuss the test.

The blast, on the 68th anniversary of North Korea's founding, drew global condemnation.

The United States said it would work with partners to impose new sanctions, and called on China to use its influence - as North Korea's main ally - to pressure the country to end its nuclear programme.

Chinese Vice Foreign Minister Zhang Yesui told North Korea's ambassador to China, Ji Jae Ryong, that he test was "not conducive to the peace and stability of the Korean peninsula", China's Foreign Ministry said.

"China urges North Korea not to take any more actions that could exacerbate tensions, and return as soon as possible to the correct direction of denuclearisation," Mr Zhang said.

The UN Security Council denounced North Korea's decision to carry out the test and said it would begin work immediately on a resolution.

The United States, Britain and France pushed for the 15-member body to impose new sanctions.

US President Barack Obama said after speaking by phone with South Korean President Park Geun-hye and with Japanese Prime Minister Shinzo Abe on Friday that they had agreed to work with the Security Council and other powers to vigorously enforce existing measures and to take "additional significant steps, including new sanctions".

Russian Foreign Minister Sergei Lavrov said it may take more than additional sanctions to resolve the crisis, suggesting that a Security Council agreement may prove difficult.

"It is too early to bury the six-party talks. We should look for ways that would allow us to resume them," Mr Lavrov said.

The talks, aimed at ending the North's nuclear programme, involve the US, Russia, Japan, South Korea, China, and North Korea, but have been defunct since 2008.