It's the latest in a flurry of similar alerts in recent days, involving nuclear power plants, trains and New York landmarks as potential targets, and the possible use of scuba divers to try to attack U.S. interests.
FBI supervisory special agent Steven Berry said Saturday the agency had issued an "intelligence update" to law enforcement regarding small planes but he declined to be more specific.
The Aircraft Owners and Pilots Association said on its Web site the alert was issued because the FBI "has received information indicating that terrorists may still be interested in using small general aviation aircraft for suicide attacks in the United States."
"Pilots are strongly encouraged to remain alert for suspicious activities anytime they are flying, or at an airport just before or after a flight," the association said. "Individuals observing anything suspicious should report it to the local FBI or law enforcement officials."
Andy Cebula, the association's senior vice president for government and technical affairs, said that "while the alert is not specific, it is important that everyone in the general aviation community serve as the eyes and ears for law enforcement, watching suspicious activity and persons."
Berry said the FBI notice was sent to law enforcement officials late in the week. "There's a lot of information that continues to come in from a variety of different sources, and we inform law enforcement or update them, in this case, of any additional information that might be useful in their efforts," he said.
The nation's nuclear power plants were placed on heightened alert as a result of information gained by the intelligence community, the Nuclear Regulatory Commission said late Friday.
The intelligence on nuclear plants did not specify a threat directed against plants or outline any plot, NRC spokeswoman Beth Hayden said, but the agency sent a special advisory to 103 plants to be cautious.
"This advisory is telling them to be on the lookout and to report anything suspicious to the operations center," Hayden said.
She noted that since Sept. 11 the nation's nuclear power plants had already been directed to increase security patrols, augment security forces, install barricades and look for suspicious people trying to conduct surveillance on the plant.
As for the scuba diver threat, the FBI said in an information bulletin issued by its National Infrastructure Protection Center that, "Recent information has determined that various terrorist elements have sought to develop an offensive scuba diver capability."
"While there is no evidence of operational planning to utilize scuba divers to carry out attacks within the United States, there is a body of information showing the desire to obtain such capability," the FBI said.
It asked the public to report any suspicious activity to local FBI offices.
An FBI spokesman said the scuba diver alert was based on "uncorroborated information" and it also was sent to state and local law enforcement agencies.
"We are sharing this information out of an abundance of caution. It is nothing to get panicked about," the spokesman said.
The scuba diver bulletin came a day after the Transportation Department warned of possible attacks on rail and transit systems across the country, again based on unconfirmed information.
Intelligence agents say terrorists may want to target U.S. subway systems, reports CBS News Correspondent Bob Orr. The threat was not specific to location or time, but was solid enough that the agents in the Department of Transportation felt that the subway systems should at least be warned.
The advisory asked transit system operators to "remain in a heightened state of alert."
DOT officials also expanded the warning to include commuter trains, Amtrak, freight railroads — all the rail infrastructure.
Because so many people will be traveling during the Memorial Day Weekend, the Transportation Department thought it would be prudent to err on the side of caution.
There has been nothing specific as to time, location or target. "This is another one of these very general threats that we have seen repeatedly since Sept. 11," said Orr.
Over the past week, a host of top U.S. officials have issued a series of warnings of possible new attacks on the United States.
Vice President Dick Cheney warned last weekend about the probability that extremists could launch fresh attacks. FBI Director Robert Mueller said on Monday another attack was "inevitable," and told President Bush this week that it would be difficult to stop another attack.
Officials said there has been a lot of intelligence coming in over the past few weeks warning of a possible attack, but they said it varied in terms of specificity and reliability.
The FBI already warned this week of possible general threats against landmarks in New York City, including the Statue of Liberty and Brooklyn Bridge.
The U.S. blames Saudi-born militant Osama bin Laden and his al Qaeda network for the Sept. 11 attacks, and a detained member of bin Laden's inner circle has been the source of many of the recent warnings.
Senior al Qaeda leader Abu Zubaydah, who was captured in Pakistan in March, has provided information recently that has led to alerts about possible threats to the landmarks in New York, apartment buildings, banks in northeastern U.S. states, supermarkets and shopping malls.
Officials acknowledge Zubaydah may not be telling the whole truth, but officials are erring on the sign of caution as they issue warnings.
There was no word on the source of the information for the warning on transit and rail systems.
But another U.S. official said it was not believed to be linked to Zubaydah.