Yesterday's agreement by major U.S. airlines to assess the safety of their foreign partners is a major step in a long-term trend toward exporting the U.S. brand of aviation safety to the world.

The step was taken reluctantly, and only under pressure from the Defense Department, which has long complained that it has no way to assess the safety of foreign airlines that haul thousands of U.S. military and civilian personnel every year.

The assessments will take place as part of a growing worldwide system among airlines called "code sharing," in which U.S. airlines share flight numbers with foreign airlines. Americans who book multiple-plane trips from U.S. airports may actually fly some legs on a foreign carrier, which will use the same flight number.

The Defense Department regularly audits the safety performance of U.S. airlines that contract to carry military personnel. But the Pentagon discovered years ago that its inspectors could not audit foreign code-share partners because the department's contract was only with the U.S. carrier, and both the State and Transportation departments cautioned that any direct effort to do so could violate principles of national sovereignty.

Yesterday, however, the Pentagon and major airlines signed a memorandum of understanding under which the U.S. carriers themselves will audit code-share safety and report the results to the Pentagon.

Mary Lou McHugh, assistant deputy undersecretary of defense for transportation policy, and Carol Hallett, president of the Air Transport Association, an aviation trade group, signed the agreement after months of negotiations involving the Defense and Transportation departments and six major airlines--American, Continen- tal, Delta, Northwest, Trans World and United.

Under the agreement, U.S. airlines will repeat the audit and report every two years. The assessment will be based on criteria set by the Defense Department and the International Civil Aviation Organization.

Although the agreement does not say so specifically, it is written so that some foreign airlines could pass the test quickly based on readily available statistics. That likely would include the major airlines of Western Europe, which have safety records equal to or better than many U.S. airlines.

But other airlines might be subjected to on-site inspections, either by the U.S. airline or by a third party chosen by the airline, such as the Flight Safety Foundation.

The agreement says that if problems are found, the U.S. airline is first obligated to work with the foreign airline to resolve them. If that fails, the Pentagon would have the right to ban use of the code-share partner for transporting military personnel.

In effect, the agreement uses the leverage of economics to force cooperation by foreign carriers. Code-share agreements are usually financially attractive to both sides.

The basic agreement was hashed out weeks ago, but airline attorneys insisted on language attempting to make clear that foreign airlines, not U.S. partners, are legally responsible for the safety of their operations.

The agreement is not the first step toward direct U.S. enforcement of safety standards outside U.S. borders, officials said.

For years, the Federal Aviation Administration has had the right to inspect and enforce safety rules for foreign airliners entering the United States. The FAA took the program a giant step forward after the crash of an Avianca jetliner in New York on Jan. 25, 1990.

The agency created a program to assess whether foreign governments had set up adequate safety oversight of their airlines. Airlines operating from any country that flunks the FAA assessment are denied landing rights in the United States. This policy avoided a direct safety assessment of the airlines by U.S. inspectors.

Prodded repeatedly by the FAA, the International Civil Aviation Organization in Montreal is in the process of setting up an international safety-assessment program.