The FBI is worried about possible terrorist attacks over the Fourth of July weekend, according to a weekly bulletin issued to law enforcement agencies and obtained by Fox News.

"Although the U.S. intelligence community possesses no specific credible threat reporting that terrorists are planning to target July 4th celebrations, we know the U.S. homeland remains a top Al Qaeda target," the report states.

It adds that the Department of Homeland Security (search) and the FBI "remain concerned about the intent of terrorists to conduct large-scale attacks in the United States."

"Al Qaeda's targets generally have been of symbolic significance, as well as directed at inflicting mass casualties, with the timing of attacks driven more by operational considerations than specific time frames," the bulletin says.

Federal, local and state law enforcement agencies "should remain vigilant to potential indicators of pre-operational planning, such as targeting infrastructure and surveillance activities."

The bulletin adds that "intelligence reporting continues to indicate a wide range of possible infrastructure targets." These include:

• Fuel farms, gas stations, pipelines and refineries

• Subways, passenger trains, freight trains carrying toxic industrial chemicals, rail and vehicle bridges and tunnels

• Financial institutions and government facilities

• Civil aviation

• Nuclear power plants

The bulletin also warns that in recent overseas attacks "terrorists have employed small-arms equipped assault teams, large vehicle-borne explosive devices and suicide bombers. These or similar tactics could be used in the United States."

"At the same time, terrorists are opportunistic and seek to exploit identified or perceived target vulnerabilities," it says. "Enhanced security measures may complicate, disrupt or negate terrorist planning."

Fox News' Anna Stolley contributed to this report