President Joe Biden’s special envoy for North Korea said Friday the United States is “preparing for all contingencies” in close coordination with its South Korean and Japanese allies as it monitors North Korean arrangements for a possible nuclear test explosion that outside officials say could be imminent.

South Korean and U.S. intelligence officials have said they detected North Korean efforts to prepare its northeastern testing ground for another nuclear test, which would be its seventh since 2006 and the first since September 2017, when it claimed to have detonated a thermonuclear bomb to fit on its intercontinental ballistic missiles.