WASHINGTON (AFP) - The United States has positioned near North Korea a destroyer capable of shooting down missiles in the latest military move amid a showdown with the communist state, an official said on Monday.

The USS Fitzgerald, which had sailed to South Korea as part of recent exercises, has been sent off the south-western coast of the Korean peninsula instead of returning to its home port in Japan, a US defence official said.

The official, speaking to AFP on condition of anonymity, said that the shifting of the USS Fitzgerald was "a prudent move" meant to offer "greater missile defense options should that become necessary."

North Korea has threatened missile attacks against the United States. The official said that the United States had earlier reduced its presence near Korean waters after the North launched a long-range rocket in December.

The December launch, which put into orbit a small satellite, set off the latest escalation cycle with North Korea. Pyongyang in February tested a nuclear bomb and has declared itself to be at a state of war with US-allied South Korea.

The United States previously took the unprecedented step of announcing test bombing by nuclear-capable state-of-the-art B-2 bombers. It has also deployed F-22 Raptor stealth fighters to South Korea as part of an ongoing exercise.

Despite the high tension, the White House said on Monday that North Korea has not backed up its threats with mass troop mobilisations or movements.