Soviet air defense units, including jet interceptors and surface-to-air missile batteries in the area where a South Korean jetliner was shot down by a Soviet fighter last week, are maintaining what U.S. officials described yesterday as a "very aggressive" level of alert and activity.

One official, noting that the United States and Japan have moved extra ships and planes into international waters southwest of the Soviet-controlled Sakhalin Island to help search for the downed airliner, said the Soviets "are fully prepared to shoot should we stray."

U.S. officials said that in recent days the Soviets almost shot down one of their planes in the same region. One official said, without disclosing the source of the information, that Soviet interceptors were prepared to fire but stopped after visually identifying the target as a Soviet plane.

The Korean Air Lines Boeing 747 carrying 269 passengers and crew members is believed to have gone down inside Soviet waters near Sakhalin, and Moscow has failed to respond to repeated U.S. and Japanese requests for permission to search for the plane there. The U.S. and Japanese search effort outside those waters has produced no confirmed sightings or recovery of wreckage, according to administration officials.

To help in the search, U.S. Air Force HC130 search aircraft and Navy P3 patrol planes are flying constantly over the region outside Soviet waters during daylight hours from Yokota and Misawa air bases in northern Japan.

The Navy has also sent a frigate, the USS Badger, and a destroyer, the USS Elliott, to assist in the search, and a Coast Guard cutter arrived in the vicinity yesterday, officials reported.

Immediately after the Korean plane was confirmed down, the Air Force dispatched an airborne warning and control system (AWACS) radar surveillance plane and five F15 fighter escorts from a base in Okinawa to Japan to assist in relaying communications from all search ships and aircraft.

Sources also said that Soviet surface-to-air missile batteries are readier for action than normal and that Soviet fighters are patrolling the region heavily.

In a statement Tuesday admitting that their planes shot down the 747, the Soviets included language that suggested to U.S. officials that the Soviets would shoot down any plane flying into their airspace, whether or not they could identify it first.

Although U.S. officials will not provide details, they claim that Soviet activities in recent days, as the search continues, reinforce that interpretation of Soviet intentions. One official said there is no indication that the Soviets have reviewed standing orders to shoot even unarmed planes if they come into their airspace.

The recent Pentagon publication on Soviet military power reported a major buildup of Soviet land, sea and air power on islands, including Sakhalin, in recent years. The interceptors involved in downing the 747 were said to be based on Sakhalin.