Japan is considering further unilateral sanctions as it condemned North Korea after Pyongyang conducted its fifth, and possibly most powerful, nuclear test yesterday.

Prime Minister Shinzo Abe called the test a flagrant violation of United Nations Security Council resolutions, and said it posed a grave challenge to international disarmament.

This year alone, North Korea has held two nuclear tests and fired 21 missiles of different types, he said, noting the "unprecedentedly quick succession" of the provocative acts.

"This suggests a marked improvement in North Korea's nuclear capabilities," he said, urging a united global response from countries including the United States, China, South Korea and Russia.

Speaking to reporters only a day earlier in Vientiane, where he attended the Asean summits, Mr Abe had said strong pressure from the international community was the only way to stop North Korea from conducting missile and nuclear tests and called for economic sanctions to be implemented "strictly".

Japan had, in February this year, in response to the January nuclear test, approved moves to cut flows of money, people and products to North Korea. It also barred North Korean ships from entering its ports and imposed a total ban on North Korean nationals entering the country, among other measures.

Mr Abe spoke to US President Barack Obama in a 10-minute phone call yesterday, during which they agreed on the need for additional sanctions.

Defence Minister Tomomi Inada, meanwhile, said the likelihood that North Korea has created a miniaturised warhead can no longer be ruled out given the "technical maturity of its nuclear weapons development".