TOKYO, Japan (AP) -- Japan toughened its guard against terrorism, boosting police agents at major railway stations in Tokyo and vowing not to back down amid reports the country could be targeted by militants.

Japan's conservative government, a firm supporter of the U.S.-led invasion of Iraq, insisted the deadly bombings that killed 200 people in Madrid this week wouldn't shake its resolve to back Washington.

But feeling vulnerable to attack, Tokyo doubled to between 450 and 500 the number of police at six major railway stations in the city, including those servicing the country's high-speed "bullet" trains.

The agents will check mysterious packages and inspect the baggage of suspicious travelers, officials said, adding that the bolstered presence was aimed specifically at preventing a Madrid-style assault.

Japan has no system of screening the luggage of passengers on the bullet trains, which travel at up to 300 kilometers per hour and carry hundreds of thousands of people a day.

The move came as the Defense Agency announced plans to form a special anti-terror and anti-guerrilla commando unit for the capital, compared in the Japanese press to the U.S. Army's "Green Beret" units. The commandos are expected to be ready this month.

Japan had already been on high alert since last month, when it tightened security at hundreds of airports, nuclear plants and government facilities as the country stepped up its dispatch of troops on a humanitarian mission in southern Iraq.

Japan's Foreign Ministry on Thursday acknowledged reports that a group claiming links to the al-Qaida terrorist network had sent a letter to a London-based Arabic-language newspaper listing Japan among countries that could be targeted by militants.

Leaders said they couldn't confirm the reports, but said such announcements wouldn't affect Japanese policy.

"Terrorist groups want to create confusion and make people worried, but we should not be swayed," Prime Minister Junichiro Koizumi said.

Chief Cabinet Secretary Yasuo Fukuda called for resolve.

"We must show that we will fight terrorism jointly with the international community," he told reporters.

It wouldn't be the first time that Japan has been listed as a target. In October, Japan was named with other U.S. allies as possible marks on a taped message attributed to al-Qaida leader Osama bin Laden.

The Spain bombings and the subsequent defeat of the pro-U.S. government in Madrid have rattled the Japanese government, which has struggled for months to rally support for the Iraq mission.

Public opinion was largely against the U.S.-led war in Iraq, and has been split over the deployment of Japanese troops to the region. The troops are strictly non-combat, and will be purifying water and carrying out other reconstruction tasks in the Iraqi city of Samawah.

The Defense Agency's anti-terror unit was to be established in a base outside of Tokyo, agency spokesman Manabu Shimamoto said Thursday. He refused to provide details on manpower, but the Mainichi newspaper reported it would have 300 members.

Shimamoto said it was possible Japan would consult other countries about training and tactics. (CNN.Com)