In an important step toward final agreement on the nuclear weapons treaty to be signed here by President Reagan and Soviet leader Mikhail Gorbachev, the Soviet Union has agreed to allow American on-site inspection of the Soviet plant where SS25 strategic nuclear missiles are made, administration officials said yesterday.

The progress on the SS25 issue was one of several advances achieved during talks in Geneva Monday and Tuesday led by U.S. chief arms negotiator Max M. Kampelman and his Soviet counterpart, Yuri Vorontsov, the officials said.

The proposed inspection, a modification of the so-called perimeter portal monitoring that has been under discussion for the last year, is the most intrusive ever considered in U.S.-Soviet negotiations. It would involve stationing U.S. personnel and devices at a gate, or portal, through which newly manufactured Soviet missiles must pass.

Other important developments, the sources said, included:A tentative accord on language that would prevent either country from circumventing the intermediate-range nuclear forces (INF) treaty, which bans medium-range and shorter-range nuclear missiles.

The Soviets had insisted on broad language which, U.S. negotiators argued, might impede existing U.S. agreements to provide nuclear weapons to allies. U.S. negotiators also feared the Soviet approach would constrain future deployment of new U.S. nuclear weapons not covered by the treaty. The United States, in contrast, favored language that would protect its alliance agreements and its future options. The two sides agreed to compromise. Soviet withdrawal of a demand that the INF treaty call for immediate follow-up negotiations, which could be aimed at reducing battlefield nuclear weapons and possibly nuclear-armed aircraft in Europe. The administration has opposed such immediate talks on grounds they would result in a drive toward "denuclearization" of Western Europe. Detailed Soviet disclosure yesterday of the number of missiles covered by the agreement that are stored in warehouses and deployed on mobile launchers.

The SS25 issue arose because the missile's first stage resembles the SS20 rocket to be banned under the INF accord. U.S. negotiators argued that monitoring the SS25 plant is needed to verify that SS20s were not being manufactured there in violation of the agreement, scheduled to be signed at next month's summit.

Many U.S. officials said they did not expect a Soviet attempt to evade the treaty by making SS20s in the SS25 plant, since the long-range SS25 is a more powerful and effective weapon. But the officials were anxious to neutralize criticism from Capitol Hill conservatives that the SS25 facility represented a treaty "loophole" and thus was a reason to oppose Senate ratification of the pact.

The Soviets have insisted that in return for such extraordinary scrutiny of the SS25, itself not covered by the treaty, the United States must permit Soviet monitoring of a facility of "comparable value" in the United States.

Sources said the two sides have not yet agreed on what U.S. facility this will be. But the sources expressed confidence that this part of the arrangement will be settled soon.

State Department spokesman Charles E. Redman, speaking to reporters several hours before Kampelman returned to Washington from the Geneva discussions yesterday, said the negotiators "did resolve a number of outstanding issues, but there are some that remain."

Other officials indicated that the two sides have not agreed, for example, on quick access to sites in Europe where treaty violations are suspected.

Redman said the U.S. side is "still confident" that the treaty will be completed in time to be signed at the Dec. 8-10 Washington summit.