Police have joined operators of major utilities and transport facilities across Japan in bolstering security measures following the government's decision Tuesday to send noncombat troops to Iraq.

Security drills and patrols were conducted near nuclear power plants in Hokkaido and Self-Defense Forces installations in Niigata Prefecture.

Nuclear plants were closed off to ordinary visitors, while other utilities, including Tokyo Gas Co., and East Japan Railway Co. have also taken various steps to improve security.

"We are on guard for whatever we can think of right now, but we must also contemplate what to do when the (Self-Defense Forces) troops actually arrive in Iraq," a senior police officer said on condition of anonymity.

The al-Qaeda terrorist network warned in November that it would carry out attacks in Tokyo if Japan sends troops to Iraq. Japan has supported the United States, its main security ally, since the early stages of the U.S.-led invasion of Iraq.

In Hokkaido, where the Tomari nuclear plant is located, prefectural police and the Japan Coast Guard conducted a joint security drill Thursday based on the premise of a terrorist attack on the plant.

Meanwhile, Niigata Prefectural Police on Wednesday began patrolling the neighborhoods of eight SDF-related facilities there.

Tottori Prefectural Police said they had tightened security checks at airports, while Aichi Prefectural Police may take steps such as installing new guardrails at walkways near designated facilities.

Meanwhile, Chugoku Electric Power Co. has suspended indefinitely the entry of ordinary visitors to the Shimane nuclear plant since Monday, at the behest of Shimane Prefectural Police.