The Soviet Union agreed today to open some of its civilian nuclear plants to international on-site inspection for the first time and suggested that such methods could be used to verify observance of arms control treaties.

The signing of a nuclear safeguards accord with the International Atomic Energy Agency was hailed by Soviet officials here as "an important and historical step" signaling Moscow's support for the 1970 nuclear nonproliferation treaty, signed by 120 nations, to prevent the spread of nuclear weapons.

But western officials said the most significant aspect of the Soviet agreement may be a new willingness to allow foreign inspection of sensitive installations. In the past, Soviet reluctance to consent to on-site inspection by outside auditors has stymied efforts to establish effective ways to monitor compliance with arms control treaties.

"This shows our country is ready for international verification in disarmament," said Vladimir Petrovsky, head of the Soviet Foreign Ministry's department for international organizations. "On the eve of U.S.-Soviet talks in Geneva we feel it is necessary to create a favorable atmosphere."

Petrovsky said, "We believe the IAEA system of safeguards represents a good, visible example of how international verification can take place when it serves the purpose of limiting nuclear weapons in general."

IAEA Director General Hans Blix, who signed the accord with Moscow, also has argued that the agency's experience and techniques could be applied to monitor future arms control agreements if its resources were expanded.

"It is not inconceivable that a complete stop or a limitation in the production of nuclear material for warheads could also be verified," Blix noted in a recent speech. "If the nuclear weapons states were looking for ways to verify the dismantling of some of their existing stocks of nuclear weapons, safeguards techniques could also be employed to verify the transfer of fissionable material to installations where it could be peacefully used or stored."

The IAEA, founded in 1957 to promote the peaceful uses of nuclear energy, carries out inspections of the nuclear fuel cycle of civilian plants to certify that fissionable material is not being diverted to military projects.

The safeguards agreement does not cover military nuclear plants and thus does not involve scrutiny of any weapons production by the five declared nuclear powers: the United States, the Soviet Union, Britain, France and China.

Those states, while not required to sign a safeguards accord since they possessed atomic weapons before the IAEA was founded, have been encouraged to set a good example to other nations by allowing inspection of their nuclear plants.

Of the five declared nuclear powers, only China now has not approved an IAEA safeguards agreement.

Andrei Petrosyants, chairman of the Soviet State Committee for Utilization of Atomic Energy, told a press conference today that "it is important to show, in view of the Geneva arms talks, that we are in favor of verifiable arms control."

Asked if Moscow was prepared to allow on-site inspection of advanced nuclear facilities and possibly military installations in the future, he said today's accord marked only the beginning of a long process.

"We hope to see the range of facilities available for inspection expanded in the future," he said.

In Washington, the State Department welcomed the Soviet action. "We believe this represents an important step by the Soviet Union, which is similar to the U.S. voluntary offer but more limited in scope," a spokesman said.

Under the safeguards accord, the Soviet Union is to submit a list of civilian nuclear plants from which IAEA inspectors will choose several for examination.

The Soviet Union operates about 40 civilian nuclear reactors and is building many more. But IAEA officials said they believe that Moscow will permit their inspectors only to look at antiquated light-water reactors and research projects rather than the modern, more sophisticated plants being erected now. In contrast, they said, the United States has recently allowed IAEA inspectors to look at highly advanced nuclear power and reprocessing plants after providing a list of all plants not involved in national security.