Two explosions were heard near Japan's Defence Ministry this evening and police said they might have been caused by radicals opposed to the dispatch of Japanese troops to Iraq.
Police said they found two steel pipes that appeared to have been used to launch projectiles from the grounds of a temple near the ministry.
A Defence Ministry official said he was unaware of any injuries or damage at the ministry, but that security officials were still making checks. The explosions occurred at about 11 p.m. (2 p.m. Irish time).
Japan has been nervous that it could become a target for attacks by Islamic militants after a report in an Arab newspaper that Osama bin Laden's al Qaeda militant network had threatened to attack Tokyo if Japanese troops set foot in Iraq.
Japan approved the dispatch of its main army contingent to help rebuild Iraq in late January, and now has about 100 troops establishing a base in Samawa, southern Iraq.
It plans to send up to 600 ground troops in a total deployment in Iraq of around 1,000, including air force and navy personnel.
Critics have argued that the deployment violates Japan's pacifist constitution. But an opinion poll in business daily Nihon Keizai yesterday said support for Prime Minister Junichiro Koizumi had risen this year, suggesting growing public acceptance of his decision to send troops to help rebuild Iraq.