China may finally have deployed multistage intercontinental ballistic missiles with a range of 3,000 to 3,500 miles, the International Institute for Strategic Studies reported yesterday.

The institute, in its annual report, said a multistage Chinese ICBM, first tested in 1976, may now be operational. The relatively limited range of the ICBM would enable it to strike targets deep within the Soviet Union, but it would not pose a threat to the Continental United States.

The report said China was also working on development of an ICBM thought to have a potential range of 8,000 miles, but that this missile was unlikely to become operational for several years.

Full-range testing of this newer ICBM, which would require impact areas in the Indian or Pacific Oceans, has not yet been carried out. The missile has been successfully used to orbit satellites, however.

The institute also reported that China has "one G-class submarine with missile launching tubes, but does not appear to have missiles for it. All the present missiles are liquid-fueled, but solid propellants are being developed."

The report said China has 30 to 40 intermediate range ballistic missiles that can reach targets 1,500 to 1,700 miles away, and 30 to 40 medium range ballistic missiles that can travel 600 to 700 miles. China was also estimated to have a stockpile of several hundred nuclear weapons.

The assessment of Peking's nuclear missile capacity was contained in a special section of the institute's annual world survey called the "Military Balance."

The authoritative, London-based research institute expressed concern in the report over the continuing Warsaw Pact military buildup, and said the Western alliance is being outclassed on land, at sea and in the air.

The institute said, however, that North Atlantic Treaty Organization defenses are still formidable and "any attempt to breach them would require a major attack.

"The overall balance still appears to make military aggression seem unattractive," the institute said.

The report also showed that the Soviet Union moved farther ahead of the United States during the year in numbers and payload of nuclear missiles.

The Military Balance said that the United States has deployed 3,600 nuclear missiles compared to 5,609 for the Soviet Union. The United States has 504 missiles with warheads in the megaton range against 1,670 for the Soviets. A nuclear megaton has an explosive force equal to one million tons of TNT.

Reporting on the land forces of the two sides, the institute said: "The Soviet Union has added 7,000 tanks of all types this year to bring its total from 43,000 to 50,000." The Warsaw Pact, the report shows, has 65,525 tanks against NATO's 25,373, including 10,500 American.

"The Warsaw Pact has also built up a marked advantage in conventional artillery . . .," the institute said.