Washington — Even as President Reagan proposes a new nuclear weapons treaty with the Soviet Union, it appears that the old treaty negotiated by the Carter administration may be unraveling.
The United States and Soviet Union are not obligated to adhere to the SALT II treaty that was signed in 1979 by President Carter and Soviet President Leonid Brezhnev. The treaty was never ratified by the US Senate.
But until recently, the Reagan administration was saying that it would informally observe the terms of the treaty as long as the Soviets did. In Senate testimony on May 11, Secretary of State Alexander M. Haig Jr. added that the two sides were in substantial compliance with the strategic arms limitation treaty (SALT II).
But other State Department officials say that the Soviets are taking actions which seem to constitute a break with the treaty.
State Department officials were reluctant to discuss what they described as ''supersensitive'' intelligence reports on new Soviet actions that could mark an intensification of the nuclear arms race. But one official did confirm that ''some recent activity'' at test sites for Soviet SS-16 missiles was disturbing to intelligence analysts. There was considerable debate over how to interpret this activity. Some analysts were reported to believe that the Soviets were using these sites to test a new type of intercontinental ballistic missile. The SS-16 missile was supposed to be banned under the SALT II treaty.
The Soviets were also reported to be encrypting -- or encoding -- information transmitted back to earth from tests of submarine-launched Typhoon NX-20 missiles. Under SALT II, this type of activity would have been prohibited, at least in cases where it impeded the verification of compliance with provisions of the treaty.
A former director of an advisory committee on arms control in the Carter administration, meanwhile, has charged that by not seeking formal approval of the SALT II treaty, the Reagan administration is actually encouraging the Soviets to ''break out'' of the nuclear weapons limits imposed by that treaty.
William E. Jackson Jr., now a guest scholar with the Brookings Institution, told the Monitor that the US now has evidence that the Soviets are developing two new ICBMS, possibly as a ''hedge'' against a breakdown in the arms-control process. The SALT II treaty allows for only one new type of ICBM, not two.
Mr. Jackson also said that the Soviets were running ''simulated tests'' of SS-18 missiles apparently designed to carry more than 10 warheads. The SALT II treaty sets a limit of 10.
In his testimony before the Senate committee, Secretary Haig said that there were ''serious flaws'' in the SALT II treaty. The Reagan administration has been particularly concerned that the treaty allows the Soviets to retain a sizable ICBM force with at least the theoretical capability of destroying the US land-based missile force in a ''first strike.'' President Reagan's new proposal would substantially reduce the Soviet stockpile of big land-based missiles.
Secretary Haig said that the US, meanwhile, must modernize its nuclear forces in order to convince the Soviets that they must negotiate a new strategic arms-control agreement along the lines of the Reagan proposal disclosed May 9.
In other developments on May 11:
* The Soviet Union's Tass news agency denounced the Reagan proposals as a maneuver designed to improve the administration's image and to camouflage its delay in formulating an arms control package.
* Former Secretary of State Cyrus R. Vance said that the administration has failed to replace harsh rhetoric with a balanced approach to the Soviet Union which emphasized US self-interest in limiting nuclear arms as well as the need to counter Soviet expansionism.
* Four former officials who helped to negotiate arms control agreements under previous administrations gave a tentative endorsement to the Reagan proposal of May 9, saying that it marked a useful first step. William G. Hyland, Jan Lodal, Joseph S. Nye Jr., and Walter Slocombe -- all members of a panel from the Carnegie Endowment for International Peace -- said they spoke as individuals and not as a panel.