Japan's naval chief of staff Koichi Furusho said Japan had confirmed that North Korea had fired a short-range anti-ship missile into the Yellow Sea after launching two similar missiles into the Sea of Japan in the past five weeks.

The announcement sparked fears that North Korea was preparing to test a longer-range ballistic missile that could hit Japan or possibly reach part of the United States.

But after a blunt denial by South Korea of any missile launch by the North, Japanese officials backtracked, saying they were still trying to confirm reports of a launch.

"We owe you many apologies for it but we make a correction," Shoei Yamanaka, chief of the defence agency's secretariat, told a press conference later.

"We are in the middle of confirming information, including the missile launch. We will have to make a clear answer based on confirmed facts," he added.

A defence agency spokesman said the information had not come from one of Japan's new spy satellites, launched last week amid strong protests from Pyongyang. Those satellites will not be operational until the system is complete with the deployment of another two, he said.

But he refused to disclose the source of the apparently erroneous information.

An agency press officer said it was not immediately certain when Japan would be able to confirm the launch or non-launch. "We cannot tell if it will be made by the end of the day."

Shortly after the initial Japanese announcement, South Korea's defense ministry issued a forceful denial that a launch had taken place.

"Based on the result of South Korean and US military intelligence authorities' investigation, we confirm again there was no missile launch from North Korea today," a spokesman said.

Military analysts in Seoul suggested that the test-firing of an artillery piece could have been confused with a missile launch.

The news on the missile firing was announced by Land, Infrastructure and Transport minister Chikage Ogi at the end of a parliamentary committee session.

"I just received a preliminary report that North Korea seems to have fired a surface-to-vessel missile at 10:15 am," she said.

She located the launch site at Hwajinri in Pyongannamdo province, on the opposite side of the Korean peninsula from the previous launch site.

The missile's range was put at 60 kilometres (38 miles).

Speculation has been rife in the Japanese media that North Korea would test-fire another missile when Japan sent its first spy satellites into orbit last Friday, a move denounced by Pyongyang as a "hostile act" that could spark a new arms race.

North Korea test-fired short-range anti-ship missiles on February 24 and March 10, sparking fresh fears in the region about its firepower.

In 1998 Pyongyang test-fired a medium-range Taepodong ballistic missile that overflew Japan and splashed down in the Pacific.

North Korea is believed to have deployed some 100 Rodong-1 missiles with a range of 1,300 kilometres (805 miles), capable of striking any target in Japan.

Analysts have said the series of short-range missile tests was part of Pyongyang's game of brinkmanship aimed at breaking US resolve during the tense stand-off over the North's nuclear weapons drive.

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