The FBI is warning U.S. law enforcement agencies of an alleged plot to bomb buses and railways in major U.S. cities later this year.

In a bulletin sent out late Thursday, the bureau said terrorists could try to hide explosives in luggage such as backpacks and duffel bags.

The bulletin cites uncorroborated evidence that the bombs could be made of diesel fuel and ammonium nitrate fertilizer, similar to what was used to blow up a U.S. government building in Oklahoma City nine years ago.

The bulletin says al-Qaida and other groups have the intent and capability to carry out such attacks, and takes note of the train bombings in Madrid last month that killed some 200 people.

In a separate bulletin sent to law enforcement agencies on Wednesday, the FBI said terrorists may attempt to enter the United States using cultural, arts, or sports visas, known as P-Visas. It says recent intelligence indicates that terrorist groups may want to exploit the State Department's P-Visa program to place operatives on U.S. soil.

Meanwhile, news media in the United States and Germany say al-Qaida has published a directive ordering supporters to target Jews, Christians, and Americans, among others.

NBC News in the United States and Germany's ZDF television reported Friday, the declaration calls on al-Qaida members to "turn the countries of blasphemers into war zones."

The 50-page document has been posted on the Internet and is signed by Abdulaziz al-Mukrin, whom both media identified as an al-Qaida leader in the Arab world. ZDF says German and U.S. intelligence officials consider the strategy paper authentic.

The paper says Jews should be the primary target, followed by Christians, Americans, Britons, Spaniards, Australians, Canadians, and Italians. It also says terrorists should focus on businessmen, diplomats, politicians, intellectuals, scientists, rabbis, missionaries, and tourists.