The U.S. embassy has formally protested to the Kremlin about a sudden, sharp, but sporadic increase in the microwave radiation beamed at the American chancery from Soviet buildings nearby.

Since September, a few weeks after a fire struck the chancery, heavily damaging its three upper floors, the Americans have recorded 26 "bursts" of heightened microwave radiation.

According to sources, the bursts have ranged in length from about five minutes up to, in one instance, an hour. Most have lasted about a half-hour.

The Soviets have bombarded the chancery for years with various levels of microwave radiation - electronic radio emissions that some U.S. scientists now say can ccuse permanent major injuries in humans.

The Soviet microwave activity became known last year in a flurry of angry exchanges between American and Soviet diplomats here and in Washington. The Soviets said the radiation was caused by powerful electronic eavesdropping equipment installed by the embassy to hear remote conversations, but the intensity of the emissions declined, and has stayed at about 2 microwatts per square centimeter ever since. This is well below U.S. industrial health standards, and about 8 microwatts below the much more stringent Soviet public health standards.

The new bursts at some occasions have reached 8.5 microwatts on the outside of the building. The radiation is beamed from across the street and from another building farther away, sources her said.

The embassy placed metal screens over all its windows last year, to filter out the radiation. Officials have said the project cut the radiation actually entering the building to "neglible" levels.

Some of the screens were damaged or destroyed in the August fire, but sources here said no one is working in Americans have theorized that the Soviets are trying to activate listening devices installed years ago inside the embassy, or to interfere with the embassy's own electronic gear, of which there apparently is a considerable amount. Some scientists now believe that sustained large doses of microwave radiation can cause depression, absent-mindedness, and other physchological problems.