The new US Secretary of State John Kerry and his South Korean counterpart have discussed how to respond, amid signs that such a test is coming. South Korea’s president also speculated that Pyongyang may detonate multiple atomic devices simultaneously.
The UN Security Council passed a resolution last month condemning a North Korean long-range rocket launch that the UN and others call a disguised test of banned missile technology.
It means Pyongyang is subject to new sanctions if it detonates its third nuclear device since 2006.
North Korea announced last month that it will conduct a nuclear test to protest against the toughened sanctions over its December launch.
During a telephone conversation between Mr Kerry and South Korean Foreign Minister Kim Sung-hwan, “there was agreement that ... if [North Korea] continues its provocative behaviour and takes further steps, that there must be further consequences,” a State Department spokeswoman said.