Many of the nation's major airlines, including United and Eastern, have canceled flights because of a growing shortage of aviation fuel.
Other airlines, such as American and Northwest, say the tightening fuel situation has not yet caused them to cancel any flights.
The airlines' actions came as, separately, Mexico's national oil company said it would "almost certainly" raise oil prices on April 1.
The Organization of Petroleum Exporting Countries plans to boost oil prices 3.9 percent on April 1, but could up that figure at a meeting scheduled for March 26.
The U.S. airlines that have been forced to cut back flights say that so far they are canceling those that are usually traveled lightly and those in which they can book passengers on other flights to the same destination.
Both the airlines and the oil companies that supply them the kerosene that is burned in jet engines say the situation varies from airport to airport and from oil company to oil company.
The shortage of aviation fuel has been triggered, in large part, because of the breakdown in crude oil production in Iran. The so-called light crude oil that Iran exports is particularly well-suited for refining into jet fuel.
Texaco Inc. announced today it would be able to sell airline customers only 80 percent of their projected requirements this month because of the shortage. Shell, United's supplier at Chicago's O'Hare Airport, told the nation's largest air carrier that it could have the same amount of jet fuel it consumed in March 1978, but no more.
Exxon, USA, however, said from its Houston base that while supplies are tight, the company so far has been able to "supply jet fuel at contractual levels."
Partly because of rising aviation fuel prices -- caused both by the Iranian shortage and the sharp increases oil-producing countries have announced in recent weeks -- airlines are expected to cut back on the use of promotional fare reductions, such as the so-called super saver fares.
The airlines will also be asking for increases which in part will reflect rising fuel costs. United, for example, has asked the Civil Aeronautics Board to approve a 1.2 percent fare increase effective May 15, while American has asked for a 4 percent increase.
Because of the fuel shortage, United said it has canceled 429 of its 12,000 flights a week. Many of the flight cancellations are at O'Hare.
United said it is trimming more flights on weekends than it is during the week, in large part because United is used heavily by businessmen.
Eastern, however, whose routes to Florida are traveled by vacationers, has concentrated its flight cutbacks during midweek to save fuel for weekends. The airline said it expects to cancel about 30 flights a day for the rest of the month.
A committee of Eastern executives meets several times each day to determine where fuel shortages are occurring and what flights could be canceled. "We're canceling lightly booked flights where we can cover our passengers" by putting them on another Eastern flight to the same destination, a spokesman said. He noted that the airline canceled a 2:36 a.m. flight from Philadelphia to Newark.
The Eastern shuttle between Washington and New York, among the airline's most traveled and most profitable runs, has not been affected, the spokesman said.
Delta Airlines has canceled 18 of its flights out of Chicago's O'Hare, while Continental Airlines has canceled four round-trip flights to Denver -- from Los Angeles, Chicago, Colorado Springs and Seattle.
In many instances the airlines are not canceling an entire flight, but what they call "legs" of flights. For example, an airline might cancel the Detroit-Chicago leg of a flight that ends up in Los Angeles, relying upon other Detroit-Chicago flights to get passengers to O'Hare.
Many airlines report they so far have not been forced to cancel flights, while one, Trans World Airlines, reported it is now back in full operation after being forced to cancel 23 flights last week, mainly because of fuel shortages in Kansas City.
Northwest Airlines said it is operating a full schedule, although it is doing more fuel "tankering," or putting more fuel than normal into a plane at an airport that has no shortage age so the plane does not have to take on fuel at an intermediate destination where aviation kerosene supplies are tight.
Airlines like to fly with as little fuel as possible, with a margin for safety, because the fuel adds to the plane's weight and reduces mileage per gallon.
American Airlines, Pan American and Braniff also report no flight cancellations because of fuel shortages.
Allegheny, whose major supplier is Texaco, said that while it has not been forced to cancel any flights, the fuel curtailment announced by Texaco today probably will force the company to cut back flights. "We don't anticipate any cancellations before the weekend," a spokesman said. Allegheny has 6,900 flights a week.