A senior member of Islamic militant group Al Qaeda has told U.S. authorities that the group had drawn up a plan to launch attacks in Japan during the 2002 World Cup, Japanese media said on Saturday.

The reports came at a time of growing concern in Japan about possible attacks on its soil linked to its decision to dispatch non-combat troops on a reconstruction mission for Iraq.

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Such concerns were fanned by reported threats late last year by Al Qaeda to "strike at the heart of Tokyo" if Japan sent troops to Iraq.

The United States has informed Japan of this information, which U.S. authorities are thought to have gained from Khalid Shaikh Mohammed, the suspected planner of the September 11, 2001 attacks on the United States, the Sankei Shimbun daily said, quoting a government source.

Shaikh Mohammed was captured in Pakistan last March and is now in U.S. custody.

Kyodo news agency carried a similar report.

Sankei said a top member of al Qaeda told U.S. authorities the attacks were never carried out because it was difficult to establish a support system because of a dearth of Muslims in Japan, which co-hosted the 2002 World Cup with South Korea.

Shaikh Mohammed has visited Japan in the past.

Sankei quoted sources as saying Shaikh Mohammed visited Japan for about three months in 1987 and received training in the use of rock-drilling machinery at a construction machinery maker in Shizuoka prefecture in central Japan.

Besides attacks within its borders, Japan is also wary of possible attacks against its ground troops dispatched to the southern Iraqi city of Samawa.

There was a scare in Samawa on Thursday when a rocket-propelled bomb caused an explosion, smashing windows in a residential area. There were no reports of injuries.

Defence Minister Shigeru Ishiba has said the attack did not seem to have been aimed at Japanese troops and it would not affect their reconstruction activities in Iraq.