SOUTH CHINA SEA (AP) _ A nuclear-powered cruiser and two other Soviet warships steamed into the South China Sea on Monday, trailed by an Australian frigate and observed by Western journalists from a U.S. Navy plane.

A Navy officer on the P-3 Orion anti-submarine patrol plane told a pool of reporters that the 28,000-ton cruiser Frunze and the 7,900-ton guided missile destroyer Osmotritelny were among three classes of warships the Soviets were bringing into the region for the first time to expand and modernize their Pacific fleet.

The Frunze is a Kirov-class cruiser with a wide variety of modern anti- submarine, anti-air and anti-surface armaments. The Osmotritelny is a Sovremenny-class destroyer with an array of cruise missiles and 130-mm guns, said the officer, who spoke on condition of anonymity. The P-3 plane took off from the Subic Bay naval base in the Philippines.

The same officer told reporters last Saturday that another type of destroyer the Soviets were bringing into the Pacific for the first time was the 8,500-ton Udaloy-class Admiral Spiridinov, which then was in the Arabian Sea. He said all three ships had been part of the Soviet Atlantic fleet.

The Orion caught up with the Soviet ships just out of the Malacca Straits in calm waters, about 300 miles north of Singapore and 240 miles south of Vietnam. The Frunze, Osmotritelny and the guided missile destroyer Strogy were in a triangle, with the Australian frigate HMS Canberra steaming in their midst.

U.S. officers on the patrol plane said they could not tell whether the Soviet flotilla, which was heading northwest, would swing north to Cam Ranh Bay in Vietnam, a former U.S. base now used by the Soviet Union, or continue their course toward the Sea of Japan and possibly the main base of the Soviet Pacific fleet at Vladivostok.

A helicopter from the Canberra hovered about 200 yards from the Osmotritelny's port bow. A Soviet helicopter was moored on the destroyer's stern and some Soviet crewmen stood on the Frunze's aft deck.

The P-3 made several passes over the ships at an altitude of 200 to 300 feet. Reporters could see 20 hatches that, according to Jane's All the World's Fighting Ships, house SSN-19 surface-to-air missiles.

''Any time there is a significant (Soviet) naval movement, we are interested in what they're doing,'' the briefing officer said, reiterating the Navy's concern over the increasing Soviet capability to control sea lanes.

He said Soviet planes also observe American ships, and made more than 100 passes ''in one day'' during U.S. naval exercises last year in the Sea of Japan and Philippine Sea.

''Obviously, it would be very welcome if Japan does build up its navy so as to protect sea lanes 1,000 miles from Japan,'' he said.

It was the second time in a year the U.S. Navy took journalists on air observation of Soviet naval movements. A similar plane flew a pool of journalists from Japan last November to observe the carrier Novorossisk.