The oil and gas industry is on especially high alert after an FBI warning that Usama bin Laden may have ordered retaliatory strikes against North American natural gas facilities in event of his capture or death, industry sources said Monday.

The sources, who spoke on condition of anonymity, said the warning issued by the FBI last week was general and singled out no specific target, but referred specifically to natural gas infrastructure such as pipelines.

There are thousands of miles of gas pipeline, most of them buried, crossing the United States and Canada. Thirty interstate gas pipelines carry 90 percent of the natural gas transported, according to the Interstate Natural Gas Association of America.

One source characterized the warning as similar to one issued earlier this month on potential attacks against West Coast bridges that prompted security alerts, but no evidence of actual terrorist intentions.

The FBI alert prompted the American Petroleum Institute, which is the lead industry group coordinating with the FBI and Energy Department on security matters, to issue a warning to oil and gas companies.

"We have received uncorroborated information that Usama bin Laden may have approved plans to attack natural gas supplies in the United States," said the memo, adding that the information was "from a source of undetermined reliability."

The FBI warning continued that "such an attack would allegedly take place in the event that either bin Laden or Taliban leader Mullah Omar are either captured or killed."

Energy companies have stepped up security at refineries, pipeline pumping stations and other facilities since the Sept. 11 terrorist attacks in New York and Washington and the U.S. retaliatory attacks in Afghanistan.

There are thousands of miles of natural gas and petroleum pipelines crossing North America, making protection of such lines difficult. Aerial monitoring of pipelines has increased and security has been intensified at pipeline pumping stations, according to industry officials, who spoke on condition of anonymity.

Also, some detailed information about location of pipelines and other energy infrastructure have been taken off some corporate and government Internet sites. Access to facilities has been tightened as well, officials said.