South Korea says there are signs that North Korea is preparing for a fourth nuclear test.

North Korea has intensified warnings in recent weeks, declaring it had entered a state of war with Seoul, threatening to strike US targets and blocking access to a border factory complex jointly run with the South.

North Korean authorities have also told embassies in Pyongyang they could not guarantee their safety from this Wednesday - saying conflict was inevitable amid joint U.S. - South Korean military exercises due to last until the end of the month.

However, no diplomats appear to have left the North Korean capital.

Tensions in the region are ratcheting up ahead of the April 15th birthday of the late Kim Il-sung, North Korea's founder.

Meanwhile, the U.S. has announced the postponement of its own missile test, in an attempt to calm relations.