Washington area hospital administrators struggled yesterday to sort out an FBI warning of an uncorroborated terrorist threat against their institutions this winter, even as a White House spokesman said the advisory had "low credibility."

The warning was provided to scores of hospitals in the Washington area, Chicago, Houston and San Francisco. In Houston, an FBI spokesman said the threat might involve explosives or anthrax, but FBI officials in Washington said it was not specific.

The FBI has issued similar advisories in recent months covering nuclear power plants, bridges, tunnels and other potential terrorist targets. In each instance, the FBI gave little direction about the appropriate security response.

Debbie Weierman, an FBI spokeswoman, said overseas intelligence sources informed the FBI that hospitals in the four urban areas "might be possible targets of terrorist actions." She emphasized that the tips "were both unsubstantiated and uncorroborated." Others familiar with the warning said it came from Pakistan but provided no details.

In the Houston area, FBI agents distributed a warning stating, "An unsubstantiated threat, from a source of unknown reliability from outside of the United States, has been received that from December 2002 to April 2003 individuals would use explosives and/or restart the anthrax crisis to do harm to a hospital."

One federal source said the FBI acted without going through "proper channels" in sharing such sketchy information. And spokesmen for the White House and its Homeland Security Office played down the newest threat.

Gordon Johndroe, a spokesman for Homeland Security Director Tom Ridge, said the threat had "low credibility."

"We do not put a lot behind this very vague threat of attacks on hospitals," Johndroe said.

The mixed messages underscored the difficulty officials have in determining how to deal with threat information.

In Washington and the other potentially targeted regions, some hospitals tightened security. Others said they already were at full readiness because they had increased security after last year's terrorist attacks and anthrax crisis.

Officials at Inova Health System, which operates five hospitals in Northern Virginia, stationed additional guards at building entrances and warned staff members to be wary of suspicious packages, according to Inova's security director, William Cody.

At Washington Hospital Center, spokesman LeRoy Tillman said, "It is a business-as-usual operation here, and, of course, our security is vigilant as is our standard practice."

Washington area hospital administrators were told of the threat Wednesday, and there was a follow-up conference call yesterday with FBI officials in which more than 40 hospital, public health and emergency managers discussed preparations and security.

Robert A. Malson, executive director of the D.C. Hospital Association, whose 17 members include Washington Hospital Center, Children's Hospital and George Washington University and Georgetown University hospitals, said participants reviewed procedures for handling suspicious packages, placing law enforcement officers in emergency rooms and handling potential anthrax exposure.

"Physically, it's very hard to take specific actions based on a nonspecific threat," Malson said, adding that administrators were told, "It would be prudent to review your protective measures and to take any actions deemed reasonable under the circumstances."

Staff writers Dan Eggen and Allan Lengel contributed to this report.