WASHINGTON (AP) _ Soviet electronic bugs secretly planted in typewriters at the U.S. Embassy in Moscow may have resulted in a serious compromise of highly classified information being handled by diplomats at the embassy, CBS News reported Monday night.

''For at least one year and probably longer, the American Embassy in Moscow was the victim of a sophisticated electronic spy operation which gave Soviet leaders an inside look at what U.S. diplomats were doing and planning,'' said CBS correspondent David Martin.

A State Department spokesman, Joseph Reap, said when asked about the CBS report, ''We do not comment on alleged intelligence activities.''

Citing unnamed ''informed sources,'' CBS said Soviet agents secretly installed tiny sensing devices in about a dozen embassy typewriters.''

''The devices picked up the contents of documents typed by embassy secretaries and transmitted them to antennas hidden in the embassy walls,'' it said. ''The antennas, in turn, relayed the signals to a listening post outside the embassy.''

The network said the bugged typewriters were in use from 1982 until the operation was uncovered in 1984.

CBS quoted one intelligence officer as saying the potential compromise of sensitive information should be viewed with ''considerable seriousness.''

''Another intelligence expert said no one knows for sure how many or what secrets were compromised. A third official called the entire affair a fiasco,'' the report said.

It added that intelligence specialists now believe Moscow was running an earlier version of the eavesdropping operation. An antenna discovered during the cleanup after a 1978 fire at the Moscow embassy ''now appears to have been part of that earlier operation,'' CBS said.

In past years, U.S. security officers assigned to Moscow have had to cope with a wide variety of Soviet snooping efforts targeted on the embassy. Electronic counterintelligence specialists conduct periodic sweeps of the embassy in an attempt to ferret out and eliminate bugging devices.

One such sweep of the embassy's south wing in 1979, for example, uncovered an array of sophisticated electronic equipment concealed in an air shaft.

According to U.S. sources familiar with that operation, KGB technicians periodically had gained access to the air shaft and surveillance devices installed there by means of a secret tunnel running from the basement of an adjoining Soviet apartment building.