A device exploded near a Japanese military ground on Sunday in what appeared to be a protest ahead of a ceremony at which the prime minister marked the 50th anniversary of Japan's modern military, media reported on Sunday.

Police later found a piece of piping, a cable and part of a shell in woods close to the Asaka training ground in Saitama, just northwest of Tokyo, Kyodo news agency said.

Prime Minister Junichiro Koizumi went ahead with a ceremonial inspection of the troops at the ground to mark the 50th anniversary of the formation of Japan's Self-Defence Forces after World War Two.

Police were investigating the explosion as a protest, Kyodo said. The blast caused no injuries,

Koizumi, a close ally of US President George W. Bush, has sent about 550 troops on a reconstruction mission to southern Iraq for their riskiest deployment since World War Two despite opposition from most Japanese.

Critics say the mission violates Japan's pacifist constitution put in place after it was defeated in 1945.

Last week's beheading of a Japanese civilian hostage in Iraq has sparked discussion on whether to extend the mission to Iraq after its mandate expires next month.

The soldiers' work has been restricted because of deteriorating security.

Left-wing groups have carried out similar small-scale attacks against government buildings and US military sites in Japan in the past. In most cases there have been no injuries or damage.

Source: CD/Agencies