The slaying of a U.S. Army major by a Soviet guard in East Germany yesterday puts a rare public spotlight on the activities of the elite intelligence units operating in East and West Germany out of military missions established by the four wartime Allied powers at the end of World War II.

"There's no question that the lives of these liaison officers revolve around being hassled," a U.S. diplomat said. "But this incident is a much more serious matter. Killing somebody is not playing by the rules."

Experts say the officers attached to each military liaison mission gather some of the best on-site intelligence to be found along the Central European front. They are usually equipped on their rounds with high-powered binoculars, infrared cameras and listening devices.

The practice of using the liaison missions as mobile military observation posts has been maintained by the four powers despite the creation since the war of two sovereign German states. The governments in Bonn and East Berlin have no official ties with the four-power missions, which are accredited only to each other.

The U.S.-Soviet accord, signed in 1947, and others signed by the Soviets with Britain and France, give each side an outpost and travel rights in the other side's occupation zones in Germany and the right to accredit 14 officers and enlisted men to their missions. The three western missions are in Potsdam, in East Germany, a few miles southwest of Berlin. The Soviets have missions in Frankfurt, Baden-Baden and in the north.

The U.S. mission maintains a white stucco villa in Potsdam, although daily patrols usually originate from West Berlin. East German border guards allow the American liaison officers free access between East and West Berlin across the Glienecke bridge. The antiquated span, where Francis Gary Powers, famed pilot of a downed U2 reconnaissance plane, was traded in 1962 for the Soviet spy Rudolf Abel, is kept free of all other traffic.

British, French and American liaison officers generally conduct daily patrols, driving all over East Germany in marked military jeeps seeking to glean insights into the nature and location of troops, missiles and armor. In West German territory, their Soviet counterparts are permitted to do the same. Only designated military zones are considered off-limits to the roving officers.

According to the 1947 agreement: "Each member of the missions will be given identical travel facilities to include identical permanent passes in the Russian and English languages, permitting complete freedom of travel wherever and whenever it will be desired over territory and roads in both zones, except places of disposition of military units, without escort or supervision."

The reconnaissance sorties, carried out by two- to four-man teams, are, nonetheless, closely monitored by Soviet Bloc authorities to thwart intrusion or snooping in restricted military areas. In the past, East German trucks have been known to bump or ram western military jeeps or fire warning shots toward them if they come too close to sensitive installations.

Such harassment has become commonplace during spring and fall maneuvers in the two Germanys, when liaison officers from the Soviet and western sides strive to learn as much as they can about the status of each others' war-fighting capability.

Last year, a three-man French military patrol was involved in a head-on collision with an East German Army truck while driving on a major road near the town of Halle. One French soldier was killed and another seriously injured in the collision, which some western diplomats said they believed was intentional. The group reportedly was trying to observe large-scale Soviet and East German maneuvers.

But such incidents rarely have evoked public controversy because of desires on both sides to minimize friction over the unusual nature of the liaison missions' work. Instead, Soviet and western military officials often try to resolve their problems in confidential meetings.

Western officials characterized the liaison officers' duties as "licensed espionage" and said it is quietly accepted that the Soviet teams who travel throughout West Germany also will try to follow maneuvers and probe into sensitive military areas.