SEOUL (AFP) - South Korea and the United States upgraded their coordinated military surveillance status on Wednesday, a report said, ahead of an expected mid-range missile launch by North Korea.

The Combined Forces Command raised the "Watchcon" status from 3 to 2 reflecting indications of a "vital threat", Yonhap said, citing a senior military official.

South Korea's military also launched an emergency task force team to monitor and analyse North Korea's preparations, the official said.

Watchcon 4 is in effect during normal peacetime, while Watchcon 3 reflects indications of an important threat.

Pyongyang is believed to have moved two Musudan missiles to its east coast last week by rail and mounted them on mobile launchers.

Yonhap on Tuesday cited South Korean intelligence as saying the North had completed preparations for the expected missile test - possibly to coincide with April 15 celebrations for the birthday of late founder Kim Il Sung.

The Korean peninsula has been locked in a cycle of escalating military tensions since the North's third nuclear test in February, which drew toughened UN sanctions.

Pyongyang's bellicose rhetoric has reached fever pitch in recent weeks, with near-daily threats of attacks on US military bases and South Korea in response to ongoing South Korean-US military exercises.

Japan, where the armed forces have been authorised to shoot down any North Korean missile headed towards its territory, said Tuesday it had deployed Patriot missiles in its capital as a pre-emptive defence measure.

A top US military commander said Tuesday that he favoured shooting down a North Korean missile only if it threatened the United States or Washington's allies in the region.