SECURITY TIGHTER AT LAX, OTHER SITES AFTER WARNING.

Byline: Daily News

Hours after a new audiotape from Osama bin Laden warned of imminent terrorist attacks against the U.S., law enforcement officials in Los Angeles said they are on alert but that the city faces no known terrorist threat.

``At this time there is no known direct threat to Los Angeles. But as always, we remain vigilant and encourage our residents to do the same,'' Mayor Antonio Villaraigosa said in a statement.

As a precaution, officials beefed up security at high-profile targets such as the Port of Los Angeles, Los Angeles International Airport and Department of Water and Power facilities, Villaraigosa said.

The Los Angeles Police Department followed through with a plan to increase its presence at LAX, which was scheduled before the release of bin Laden's threat, Officer Maria Garcia said.

``We're always keeping a watchful eye,'' Garcia said.

The FBI's Los Angeles field office made no changes as a result of the tape, FBI spokeswoman Vickie Hampton-Franklin said.

``We are continuing to investigate leads that we get, gather information, things of that nature,'' Hampton-Franklin said.

In the tape, which was aired in part on Al-Jazeera television Thursday morning and in whole on Al-Jazeera's Web site, bin Laden points to July's bombing in London's subway system, and says similar attacks in the United States are imminent.

``The delay in similar operations happening in America has not been because of failure to break through your security measures,'' bin Laden said, according to a translation by The Associated Press. ``The operations are under preparation and you will see them in your homes the minute they are through (with preparations), with God's permission.''