"South Korean and US military authorities have recently noticed missile-related activities in North Korea," spokesman Nam Dae-Yeon of the defence ministry said.

"The North's missile-related activities are probably part of its annual military drill," he said.

"We cannot rule out the possibility of the North test-firing missiles. South Korean and US authorities are closely following the movement in the North," Nam said.

"It's not certain whether North Korea is now conducting an annual military drill, but it is highly likely that the movement is part of an annual military exercise," he said.

Yonhap news agency quoted an intelligence source as saying that military vehicles, soldiers and missile experts were converging around an unidentified launch site of North Korea's intermediate range Rodong missile.

"Taking various possibilities into account, the possibility of the North actually test-firing missiles is not so high. It seems that this is merely part of a routine drill by North Korean missile units," the source was quoted as saying.

The same intelligence source told Yonhap that the test was more likely to be a command post simulation rather than a real test-firing.

He said two years earlier during similar drills North Korea carried out test-firing simulations on Scuds and Rodongs.

In Tokyo, the daily Yomiuri Shimbun said US and Japanese authorities had determined from satellite images and radio traffic that North Korea was preparing to launch a Rodong ballistic missile with the range of 1,300 kilometres (800 miles).

Japan's Defence Agency consequently sent an Aegis-equipped destroyer to the Sea of Japan (East Sea), the daily said.

Another destroyer is also in the Japan Sea while an EP-3 electronic reconnaissance aircraft is hovering above to keep round-the-clock surveillance, Yomiuri quoted anonymous government sources as saying.

No defence agency officials were available Thursday to confirm the report.

Military experts believe it would take several days to two weeks even if North Korea goes ahead with an actual launch, the paper added.

A Rodong, with the range of some 1,300 kilometres, can hit almost all of the Japanese archipelago.

In August 1998, Pyongyang stunned the world by test-launching a Taepodong missile over Japan, officially claiming it was a satellite launch.

The Yomiuri said there was also a possibility of a Taepodong missile launch as Rodong bases are surrounded by underground silos containing Taepodong-II ballistic missiles with a range of 3,500-6,000 kilometres.

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