Soviet-made SA5 antiaircraft missiles being deployed in Libya are likely to be operational this month, far sooner than U.S. officials first believed, and another shipment of the long-range missiles may recently have reached the North African country, U.S. officials said yesterday.

Twelve SA5 missile launchers are being erected at a fighter air base on the Gulf of Sidra, which Libya claims as territorial waters but the United States considers international territory.

From their location at the Ghurdabiyah base, the missiles could reach targets over most of the Gulf, where U.S. and Libyan forces have tangled in the past.

When the missiles were first spotted late last year, U.S. officials said they would probably not be operational for at least a year. Syria, the only other nation outside the East bloc to have received SA5s, spent about 20 months putting them in place.

The Libyans have been working hurriedly to get the antiaircraft missiles at least marginally operational and have protected the site with shorter-range antiaircraft missiles, officials said.

Although U.S. officials still believe that the SA5s will not be fully operational with their component radars soon, the Libyans appear to be "jury-rigging" the missiles with less capable radars, they said.

Deployment of the missiles has increased tension between Tripoli and Washington, which has accused the Libyans of complicity in recent terrorist attacks at the Rome and Vienna airports. Some officials have called for a U.S. air raid against terrorist training sites in Libya, a mission that could be complicated by the SA5s.

The SA5, with a range of almost 200 miles and a maximum altitude of about 100,000 feet, has a semiactive homing system, meaning it could chase target aircraft.

The missile is too slow in most situations to hit jet fighters, which are fast and relatively small, but it could threaten larger planes that might operate over the Gulf of Sidra. These include P3 submarine hunters or Airborne Warning and Control System (AWACS) reconnaissance planes.

The Soviets have built three versions of the SA5, and the Libyans have received the second variant -- not the state-of-the-art equipment but not the virtual antique that first appeared in the 1960s, officials said.

U.S. officials first expected the Libyans to erect six missile launchers apiece at two sites, one on each end of the Gulf. Instead, all 12 appear to be going up at the same location, just southeast of the town of Sidra.

But intelligence officials said they believe that they spotted another shipment of SA5 missiles, which could be destined for a second site, leaving the Black Sea for Libya recently.

Libyans have been training in the Soviet Union to operate SA5s since late last summer, and U.S. officials said they believe that most will operate the weapons without Sov- iet help, following their usual pattern.

Some Soviets are involved in deployment of the Libyan SA5s, but not the large Soviet contingents that help the Syrians maintain theirs.

Two U.S. fighter jets, carrier-based F14 Tomcats, shot down two Libyan fighters over the Gulf of Sidra after a brief dogfight in August 1981.

Since then, the Libyans have not challenged U.S. Navy aircraft that have ventured over the Gulf several times in what the Navy calls "freedom of navigation" exercises -- deliberate maneuvers to demonstrate that the United States does not accept Libyan claims to that portion of the Mediterranean Sea.