One of the hard-fought provisions of SALT II was that each side could flight-test and deploy only one additional "new type" of land-based intercontinental ballistic missile during the life of the treaty.

In 1982, the Soviets began to test a new weapon dubbed the SS24; Moscow notified the United States that this would be the "new type" permitted under SALT II. In early 1983, the Soviets began testing a second new weapon, which the United States calls the SS25 and which is a violation of the limits, according to the U.S. government.

The U.S. objective in limiting "new types" of missiles was to impede the race in quality and effectiveness of strategic weapons, a race as serious as that in numbers of weapons. Ralph Earle, who was part of the U.S. negotiating team during the SALT II process, said recently that the restriction on new types of missiles in the treaty was only "a gesture" toward limits on quality that Washington had hoped to obtain.

The Carter administration initially proposed a ban on any "new types" of ICBMs but the Soviets refused. Eventually the two sides agreed on one new missile each, so the United States could go ahead with its planned MX missile and the Soviets with either a new multiwarhead weapon (such as the SS24) or a new single-warhead weapon (such as the SS25). The Reagan administration contends that the Soviets went ahead with both in violation of the treaty.

How to distinguish a "new type" from an existing missile was the subject of much negotiation. In the end the two sides agreed to consider up to a 5 percent variation in length, diameter, launch-weight or throw-weight as a modification of an existing weapon; above 5 percent would be considered a "new type."

The Soviet Union has claimed that the SS25 is a permissible modernization of the SS13, an old single-warhead weapon from the 1960s. Rejecting this claim, the U.S. Arms Control and Disarmament Agency said recently the throw-weight of the SS25 is 50 percent greater than the SS13 and, thus, far from what is allowed.

Marshal Sergei Akhromeyev, Soviet army chief of staff, said this week that the United States has underestimated the throw-weight of the SS13 and overestimated that of the SS25, citing technicalities that the United States rejects.

Since the Soviets will not disclose the specifications of their weapons, the United States relies on calculations based on observation of Soviet missiles in test flights and interception of missile test data, known as telemetry, which is radioed to Earth.

Secretary of Defense Caspar W. Weinberger said June 1 that 72 SS25 missiles have been deployed, "each one a violation of the SALT agreement." The disputed weapon is being deployed atop a truck for easy mobility. This would make the SS25 less vulnerable to U.S. attack in time of war and particularly valuable if fixed, silo-based Soviet missiles are threatened or knocked out.

The United States is in the early stages of developing a second "new type" of missile, the single-warhead Midgetman. Officials said this is not a violation of the treaty now because it is far from flight-testing stage.