Following the outbreak of war in the Middle East, most U.S. airlines are relaxing their cancellation policies, airports have tightened security procedures dramatically and terrorism experts are recommending that travelers reconsider plans to go abroad. The State Department also has urged travelers to keep abreast of current official advisories. More specifically:

Most airlines in this country are allowing passengers to change itineraries or cancel flights without paying the customary penalty imposed on discounted fares. As of Friday night, however, surprisingly few people had taken them up on the offer, according to the carriers, although the number of cancellations on transatlantic flights was growing.

Air travelers are being advised to arrive at the airport early to give themselves plenty of time to check in. Security procedures for check-in and boarding have been tightened, and passengers may encounter delays. United Airlines is encouraging passengers to show up two hours in advance for both domestic flights and international flights.

International travelers should be aware that a number of State Department advisories remain in effect cautioning Americans to defer or exercise care when traveling in certain countries in the Middle East, North Africa and South Asia.

Security expert Peter Savage of Baltimore and other experts on terrorism are advising travelers to postpone transatlantic travel if they can until the Middle East situation settles down -- or at least to avoid such high-profile transportation hubs as London, Paris, Frankfurt, Madrid and Athens. Savage is a consultant on travel risks for business executives and the author of "The Safe Travel Book," a guide for international travelers.

However, Savage points out that air carriers have become more difficult targets for terrorist attack. The conflict in the Middle East has heightened security awareness, and strict new procedures for examining checked luggage have been in effect for some time.

Any American traveler abroad, he says, should try to remain inconspicuous so as not to draw attention. Defense contractors, government officials and other "high-profile" travelers should examine their wallets and carry-on luggage for any items such as business cards and letterhead stationery that might link them to the government or military should a plane be hijacked. These items should be packed in checked luggage.

Among the new developments affecting travelers:

Airline Policies: Federal officials have issued warnings about the potential for terrorism attacks both in this country and abroad. Some travelers apparently are cutting trips short to return home, and others are telling the airlines they want to postpone or cancel trips while hostilities continue in the Middle East. A number of U.S. firms reportedly have sharply curtailed business travel -- both domestic and international -- asking employees to use the phone or fax machines instead until the Middle East situation settles.

As a result, most U.S. airlines have adopted varying policies permitting itinerary changes and cancellations. USAir, which appears to have one of the most lenient polices, is waiving all penalties for changes or cancellations in both domestic and international travel. Any unused portion of a ticket, says spokesman David Shipley, can be used as credit for a ticket at a later date.

Among other airlines, passengers on United who already have left home can change their itinerary or return home early without penalty, according to spokeswoman Sara Dornacker. Passengers who have not begun their trip can exchange a domestic or international ticket for a voucher good for a year.

Pan Am and TWA are limiting changes and cancellations to transatlantic flights. They will permit itinerary changes without penalty and will issue vouchers good for a year for cancellations. Northwest will allow changes and cancellations (covered by vouchers) for transpacific and domestic flights, as well as for transatlantic flights. Delta Air Lines will consider requests for a change or cancellation on a "case-by-case" basis, says spokesman Neil Monroe.

Northwest and TWA say the relaxation of penalties will remain in effect through the end of January. Pan Am's policy remains in effect until Jan. 24. Other airlines have set no specific date, and any of the expiration dates can change as events develop in the Middle East. If you hold tickets you want to change, consult the airline for the latest information.

So far, only a few travelers appear to be taking the airlines up on their offer. "United is getting very few such calls for domestic flights," says Joe Hopkins, but "there has been some erosion to Europe." Pan Am spokesman Alan Loftin says the cancellation rate in the wake of the hostilities is about the same as it was in mid-January last year. Northwest's Christy Clapp says the airline has had only a "small" number of requests for change.

Airport Security: Travelers using Washington-National and Washington-Dulles International will notice heightened security measures immediately on arrival. For one thing, the Federal Aviation Administration has suspended curbside check-in at all U.S. airports. Skycaps are available, but they can only assist in carrying luggage to a check-in desk inside the airport. Lines at check-in counters may be longer than usual.

United estimates that about 21 percent of its passengers normally take advantage of curbside check-in.

In addition, only passengers holding tickets will be permitted into boarding areas beyond the X-ray and metal-detecting devices in any U.S. airport. If you are expecting to be met at your destination, look for your party outside the security area. Airports also have been told to strictly enforce the ban against leaving automobiles parked unattended outside airport entranceways. They will be towed.

Explosive detection dogs will be used "where appropriate," says the FAA, and guards will make more intensive inspections of restrooms, telephone booths, trash receptacles and other public areas.

At both National and Dulles, security guards are taking extra care to check passengers and carry-on luggage, according to spokesman Dave Hess of the Metropolitan Washington Airports Authority. Delays may result, so you should heed the advice to give yourself plenty of time -- especially for international flights.

Security Precautions: In a press conference Friday, the FAA advised travelers to take the following precautions:

Carry less luggage and don't leave it unattended.

Report any unattended bag you see to a security guard.

Avoid carrying any large electric or electronic devices since they may need to be inspected.

Don't carry bags onto a plane for others, and don't check in someone else's bags.

Report suspicious activity to a guard.

State Department Advisories: In the days preceding the Jan. 15 deadline for Iraqi withdrawal from Kuwait, the U.S. State Department issued a series of warnings advising Americans to defer travel or use caution when visiting a number of countries in the Middle East, North Africa and South Asia. These warnings remain in effect, and new ones have been added as a result of the outbreak of hositilities. Before going abroad, check for the most up-to-date official advisories with the Citizens Emergency Center, 202-647-5225.

The State Department says Americans should consider deferring all travel to Israel, Sudan, Algeria, Jordan, Mauritania, Morocco, Tunisia, Pakistan, Yemen, Bangladesh, Djibouti, Qatar, United Arab Emirates, the east province of Saudi Arabia, Syria and northern Nigeria. Demonstrations, terrorist attacks or other hostile acts may be directed against the U.S. government or American citizens in some of these countries. Israel has been the target of Iraqi missiles.

The State Department adds that terrorism against Americans is a possibility at other locations throughout the world. It urges travelers to keep in contact with the nearest U.S. Embassy or consulate for information.

Airline Bankruptcy and Frequent-Flier Miles Should frequent fliers who have accumulated substantial mileage in airline bonus programs be worried about losing them when an airline files for bankruptcy, as Pan Am has just done? The question was phoned in last week by a concerned traveler, a Washington man with lots of Pan Am mileage credits, who wanted to know if he should start using them as rapidly as possible.

Determining what's going to happen next in the troubled U.S. airline industry is not an enviable task. But Randy Petersen, whose profession it is to keep a close eye on airline bonus programs, says his advice is to hold onto the miles. "I think at this point it would be foolish to go out and try and burn them off quickly."

Petersen is the editor of Frequent, a newsletter aimed at keeping subscribers abreast of developments in frequent-traveler programs. He also happens to have banked about 600,000 miles in Pan Am's frequent-flier program and says, "I'm not worried."

Petersen offers the same advice to frequent fliers on Continental Airlines, which also has filed for bankruptcy.

His optimism is based on two factors. If a financially stronger airline acquires any of the carriers, it almost certainly will honor the carrier's frequent-flier program. If it doesn't, he says, it would alienate the well-traveled passengers it would hope to attract. In previous airline acquisitions, the precedent has been to retain the programs.

And, he notes, both Pan Am and Continental still have valuable assets that make it improbable either would disappear, as Braniff did a while back, without another airline acquiring them. Eastern's bonus miles are good on Continental flights, and presumably they would be on any airline that might buy Continental or merge with it.

Meanwhile, both Pan Am and Continental are flying full schedules and both say their mileage programs remain valid. United Airlines, which is in the process of buying Pan Am's valuable London routes, says it expects to honor Pan Am miles.

Sara Dornacker, a United spokeswoman, says the marketing agreement with Pan Am includes a provision that travelers who are members of United's Mileage Plus or Pan Am's Worldpass programs can earn and redeem awards on both carriers. Details currently are being worked out by a task force representing both airlines. The new program will be announced if and when the necessary approval for the sale is obtained from the British government.

"We've been trying to get the message across," says Pan Am spokeswoman Elizabeth Hlinko, "it's business as usual here." She adds that if the sale to United goes through, as expected, Pan Am's mileage program actually will be enhanced.

"Although Continental is undergoing a financial reorganization," says spokesman David Messing, "there's no question that it's going to be a player in the airline industry for the long term." This means anyone holding Continental or Eastern mileage, he says, "can have faith they can use it."

Some passengers have contacted Continental about the safety of their One Pass mileage accounts. The problem, says Messing, is that they may be associating the word "bankruptcy" with going out of business. "That's not the case with Continental."

Olympic Packages The 1992 Olympic Games are more than a year away, but Olson-Travelworld of El Segundo, Calif., the exclusive U.S. ticket agent, has just published two 15-page brochures describing a variety of five- to 21-day tour packages. One brochure features the Winter Games in Albertville, France, which will be Feb. 8 to 23, 1992, and the other covers the Summer Games in Barcelona, July 25 to Aug. 9, 1992.

Order forms for tickets to specific competitive events at both Olympics will be available from Olson-Travelworld next month. The forms will be mailed automatically to anyone who has requested brochures, but travelers not interested in the tours also can request the ticket forms.

The packages -- which are expensive -- offer options for air travel, lodgings and extended sightseeing tours before and after the Games. The cost includes lodging, daily breakfast, airport transfers and transportation between hotels and the numerous scattered sites where events will be held. Prices begin at $2,290 per person (double occupancy) for a five-night Winter Games package and $2,790 for a six-night Summer Games package.

The firm guarantees that if it cannot fill at least 50 percent of a traveler's requests for event tickets, it will refund payments made for tour packages.

Tickets for oversubscribed events, including opening and closing ceremonies, will be distributed by a computerized lottery system.

For brochures or ticket order forms: Olson-Travelworld, 100 N. Sepulveda Blvd., El Segundo, Calif. 90245, 800-874-1992.