The United States and the Soviet Union yesterday concluded an unprecedented three days of talks on nuclear nonproliferation after agreeing on the importance of strengthening the monitoring role of the International Atomic Energy Agency, according to sources.

The two superpowers, during sessions that began Wednesday at the State Department, discussed the nuclear activities of a number of countries that are viewed as potentially interested in acquiring atomic weapons.

"But the major issue on the minds of the Soviets was our current reassessment of the role of the atomic energy agency," a participant said.

The United States walked out of the annual meeting of the agency in September after the organization rejected the credentials of Israel, and the Reagan administration has been boycotting the agency while conducting a "reassessment" of future U.S. participation.

While the major U.S. concern -- stressed once again to the Soviets this week -- has been the growing politicization of the agency, the administration also has been eager to see a strengthening of the division of the agency charged with "safeguarding" civilian nuclear installations.

The United States has long felt that the agency viewed its "safeguards" activities, which are designed to provide timely warning of the diversion of material from civilian nuclear facilities for possible use in a weapons program, as less important than the promotion of atomic power.

But the Soviets, who generally share American nonproliferation concerns, fear that the U.S. boycott, if continued much longer, could permanently weaken the agency.

The administration reassured the Soviets that its goal is to "reinvigorate" the agency, sources said, and indicated that the administration will complete the reassessment and announce its decision to resume participation in the agency's activities by the end of January.