The United States may launch unilateral sanctions against North Korea, a US special envoy for the isolated state said on Sunday, two days after it carried out its fifth and biggest nuclear test in defiance of UN sanctions.

"In addition to action in the Security Council, both the US and Japan, together with the Republic of Korea, will be looking at unilateral measures, as well as bilateral measures, as well as possible trilateral cooperation," said Sung Kim, the envoy, referring to South Korea by its official name.

Specific details of the US unilateral sanctions have yet to be decided, Kim said, speaking to reporters in Tokyo after meeting Japanese foreign ministry officials.

But both the US and Japan were looking at "a full range of possibilities, in terms of additional unilateral sanctions that can be implemented," he added.

North Korea on Friday set off its most powerful nuclear explosion to date, saying it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that its rivals and the United Nations have been powerless to contain.

The UN Security Council denounced North Korea's decision to carry out the test and said it would immediately begin working on a resolution. The US, Britain and France pushed for the 15-member body to impose new sanctions.

"We will be working very closely in the Security Council and beyond to come up with the strongest possible measure against North Korea's latest actions," said Kim.