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.
Kanasugi said Seoul, Tokyo and Washington would be coordinating their response.
"We agreed to continue Japan-US and Japan-US-South Korea cooperation...as we work toward an adoption of a new UN Security Council sanction that will include further sanction measures against North Korea," he said in a statement.
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