The city government of Tokyo has held a drill to prepare for a hypothetical biological attack.
Around 170 officials from the Tokyo city government and fire department participated in a role-playing drill on Monday, an official for the Tokyo Metropolitan government's bureau of general affairs said.
"The premise is that the smallpox virus has been released in a Tokyo subway," he said, adding that the drill was being held on a floor of the city government offices, with participants gathered in different rooms and communicating via fax and phone.
Recent media reports have said that Osama bin Laden's al-Qaeda had warned Japan it would strike at the heart of Tokyo if Japan sent troops to help with the rebuilding of Iraq, but the official said the drill was not held in response to that.
In March 1995 members of the Aum Supreme Truth cult released the deadly nerve gas sarin on crowded subway trains in Tokyo, killing 12 people and making thousands ill.
The attack by the members of the cult, which believed that the world was coming to an end, shocked a nation accustomed to relatively crime-free streets.