Security checks were tightened Monday on visitors to the Prime Minister's Official Residence amid rising concern over terrorist attacks both at home and abroad.

Metal detectors and X-ray scanners for visitors were installed at the entrance gate, and the front main doors to the building were kept closed.

All entrants, including Prime Minister Junichiro Koizumi and members of his Cabinet, are now required to go out through side doors.

Government officials said the measures are due to a rising threat of terrorist attacks. Japan has dispatched troops to Iraq on a noncombat humanitarian mission, joining military forces from other countries supporting the U.S.-led occupation.

The main entrance was closed to prevent terrorists from ramming vehicles into the building, they said.

The closure of the main entrance also deprives reporters of staging spontaneous, on-the-spot interviews with officials as they wait for their vehicles outside the main entrance.

"The Prime Minister's Official Residence is the center of central government administration," Chief Cabinet Secretary Yasuo Fukuda said during his daily news conference. "We have conducted various studies, but concluded there is no other way to ensure security."

Cabinet members, vice ministers, Diet members, national guests and reporters regularly stationed at the official residence who have special ID cards will be exempt from metal detector and X-ray checks of their belongings, the government officials said.

"It's difficult to say when we will terminate these measures," Fukuda said. "We'll make a decision depending on the situation."