The government has received information that 12 foreign Islamic extremists may have entered Japan shortly before last week's terrorist attacks in the United States, informed sources said Monday.

The information was provided by a foreign intelligence agency, the sources said.

Although the tipoff has not been verified, the National Police Agency has directed police nationwide to beef up security around U.S. military bases and other U.S. facilities.

Japanese immigration authorities are also examining flight records at Narita, Kansai and other airports that handle international flights for indications of their presence, the sources said.

Public security authorities said Japan was informed that the individuals in question entered the country mainly via Pakistan sometime in early September, just ahead of the Sept. 11 terrorist attacks in New York and Washington.

They are believed to be of Arab origin and followers of Saudi exile Osama bin Laden, whom the U.S. regards as "the prime suspect" in the attacks on the World Trade Center and the Pentagon.

Authorities are checking with key railway stations, hotels and airports across the country as part of their investigations. They believe the people in question may have entered the country using counterfeit passports, the sources added.

On Sept. 7, the U.S. Embassy in Tokyo warned that U.S. military installations in Japan and South Korea may be terrorist targets.

Japanese leave Afghanistan

Japanese nationals still in Afghanistan and nearby countries that are subject to upgraded travel advisories from the Foreign Ministry are leaving the areas, ministry officials said Monday.

The ministry has also ordered its overseas offices to reinforce safety-confirmation measures, the officials said.

Of the 15 Japanese in Afghanistan until recently, only three -- two journalists and a Catholic priest -- were still there as of Monday morning. However, they are making their way out or planning to do so, the officials said.

Saudi dissident Osama bin Laden, named by U.S. President George W. Bush as the prime suspect behind the terrorist attacks in New York and Washington last week, is reportedly hiding in Afghanistan.

Bush has expressed his resolve to hunt down those responsible for the Sept. 11 attacks and take retaliatory action. He said the United States is "at war" with those who staged the devastating terror attacks and pledged to stage a broad and sustained campaign against "the evil of terrorism."

On Sunday evening, the ministry upgraded to level four its travel advisory to Japanese nationals for Pakistan, Baghdad and other parts of Iraq, and areas within 50 km of Afghanistan's borders. The ministry urged Japanese to prepare to leave the areas and have accompanying family members move to safe locations.

The travel advisory for Afghanistan remains at the highest level of five, which warns all Japanese to get out.

Nearly 300 of the more than 800 Japanese who had until recently been in Pakistan have left, the officials said.

The number of Japanese missing since the terrorist attacks in the U.S. remains at 24. Twenty-two of them are believed to have been working at offices of companies housed in the twin towers of New York's World Trade Center when the structures were razed by hijacked planes.

Of the two remaining missing, one is believed to have been on board one of the jetliners that slammed into the World Trade Center and another on the airplane that crashed near Pittsburgh.

The Foreign Ministry has also been unable to confirm the safety of about 70 to 80 out of about 600 Japanese whose families have asked the government to look into their whereabouts following last week's terrorist attacks. The 22 believed to have been inside the World Trade Center are not included.

The ministry issued orders Sunday to its overseas establishments, instructing them to review the safety situations of the host countries and gather information about the movements of Islamic extremists in the countries, the officials said.

The diplomats were also told to reconfirm the crisis management system for Japanese nationals as well as the escape plans for them in the event of an emergency, and to give accurate advice to Japanese in the respective countries where they are serving.