WASHINGTON (AP) _ Airports across the nation were ordered into the highest state of security since the Persian Gulf War on Sunday as a defense against possible terrorist attacks.

Secretary of Transportation Federico Pena didn't cite any specific threats in issuing the order, saying only, ``Based on several current and future events, more security measures are now warranted.''

But a department official said the increased security was related to several sensitive events _ a group of militant Muslims being found guilty Sunday in a New York terrorism trial, the signing of a Mideast peace treaty last week, the visit this week by Pope John Paul II and the United Nations' 50th anniversary celebration later this month.

``There is a general level of possible threats that we're vulnerable to because of several things,'' said the official, who spoke on the condition that he remain anonymous. ``We're not seeing threats related only to transportation issues, either. I would expect within the next few days or next few weeks to see a general tightening of security everywhere.''

The stricter airport security measures could include closing off rooftop parking lots, searching cars, towing vehicles left unattended outside terminals, asking for picture identification of suspicious persons, searching baggage, and ending non-passenger access to certain airport areas.

Tara Hamilton, a spokeswoman for the National and Dulles International airports just outside of Washington, said passengers shouldn't expect many delays because people already seem used to stepped up security. Most international passengers arrive two hours ahead of scheduled flights and domestic travelers now are counseled to arrive one hour before takeoff.

``What people might notice is an increase in surveillance by airport police,'' Hamilton said. ``And we'll have increased announcements over the (public address) system reminding people to keep an eye on their luggage...This is all in place, but it will be stepped up.''

At Chicago's Midway and O'Hare International Airport beginning Wednesday passengers must be prepared to show a government-issued identification such as a driver's license or passport, and searches are possible.

``Other than that, everything else is not discernible to the public,'' said Lisa Howard, a spokeswoman for Chicago's Aviation Department. ``These measures are not as stringent as what we saw with the Persian Gulf War, which allowed ticketed passengers only and had no skycap check-in.''

The Clinton administration began a security crackdown at federal buildings in the spring after the April 19 bombing of a federal building in Oklahoma City killed 169 people. Two Americans have been charged in that explosion.

The bombing led to widespread heightening of security around federal facilities. It helped prompt the decision to block vehicle traffic on Pennsylvania Avenue outside the White House.

On Aug. 9, Pena ordered a ``heightened state of alert'' for transportation facilities nationwide after President Clinton called for a review of airport security procedures.

The administration refused to reveal details of any threats, but Newsday reported that the FBI had received intelligence that two militant groups _ the Palestinian Hamas and the Iran-backed Hezbollah, or Party of God _ were planning a ``suicide massacre.'' The reported threats appeared related to the then-ongoing terrorist conspiracy trial of Sheik Omar Abdel-Rahman and nine other militant Muslims and to U.S. plans to extradite a reputed Hamas leader, Mousa Abu Marzuk, to Israel.

On Sunday, Pena announced that he has asked Federal Aviation Administrator David Hinson to direct airport authorities and air carriers within the United states to begin implementing the higher security rules as outlined in a ``directive'' issued by the FAA in early August.

``The decision to increase security is based upon an assessment provided by federal law enforcement and intelligence agencies, combined with an analysis of the current state of affairs,'' Pena said in a statement.

``There has been no specific threat to aviation or any other form of transportation, but we are dealing with a general threat against all U.S. interests at home and abroad.''

Pena said travelers could help avoid delays by:

_Clearly labeling luggage and maintaining control of bags.

_Observing parking and unloading restrictions.

_Remaining alert for any suspicious, unattended bags, parcels or other items and reporting them to security.

_Being prepared to show identification and answer questions about baggage.