US, Japan scramble for 'strongest' sanctions against North Korea
Sung Kim, the US State Department's special representative for North Korea policy, also suggested that the US may launch its own sanctions in response to "the provocative and unacceptable behaviour by the North Koreans."
"We will be working together very closely in the Security Council and beyond to come up the strongest possible measure against North Korea's latest action," Kim told reporters in Tokyo after meeting his Japanese counterpart Kenji Kanasugi.
"In addition to sanctions in the Security Council, both the US and Japan, together with the ROK (South Korea), we will be looking at unilateral measures," Kim said, without going into further detail.
North Korea has been hit by five sets of UN sanctions since it first tested a nuclear device in 2006, but has insisted it will continue, come what may.
The North carried out its fifth nuclear test on Friday, claiming that it had successfully tested a nuclear warhead, and drawing global condemnation.
The international community has engaged in a flurry of diplomacy in an attempt to persuade China to use its leverage to persuade Pyongyang to comply with UN sanction resolutions.
China has said it "firmly opposes" the test, but analysts believe Beijing wants to avoid a collapse of North Korea in order to prevent the balance of power on the Korean peninsula from leaning towards the US.
Washington's "dialogue" with Beijing over the crisis will continue, Kim said.
"We continue to work together to urge China to implement existing Security Council resolutions...and to work with us to make sure North Korea's behaviour and their deliberation change in a more productive and positive direction," Kim said.
"North Korea continues to present growing threats to the region, to our allies and to ourselves. We will do everything possible to defend against that growing threat," he said.