Twenty months after it was first suspected that Pakistan might be secretly diverting plutonium from its atomic power plant for use in nuclear weapons, the Pakistani government has agreed to most of the new safeguards needed to detect future diversions, it was disclosed yesterday.

But Dr. Hans Blix, director-general of the International Atomic Energy Agency, said in an interview: "We still have at least one item to settle with the Pakistanis before we would be in a position to give assurances" that Pakistan is not diverting plutonium from the civilian reactor outside Karachi to its weapons program.

Although Blix declined to amplify on the remaining problem, U.S. sources said the international agency probably was insisting on placing an inspection seal on an access hatch that could be used to covertly remove material from the containment around the Karachi reactor. The "port" presently is not sealed because it was not in the plant's original blueprints.

If agreement on the remaining step can be reached, Blix said he hoped to report to the atomic agency's board at its next meeting in February that "we soon will be in a position to give assurance" that no diversions are taking place.

But U.S. sources said the Pakistanis, who have been operating the reactor for the past year and a half at a reduced power level that is ideal for producing weapons-grade plutonium, presumably could have produced 10 to 20 kilograms of plutonium in that time -- enough material for one or two nuclear bombs.

Blix said Pakistan already had accepted "quite a number of measures" proposed by the atomic agency, including the readjustment of television cameras installed by the agency two months ago to "give us a better view" of critical areas.

Pakistan also has agreed to the installation next month of a new "bundle counter" that automatically keeps track of how many bundles of irradiated fuel elements containing plutonium have been removed from the reactor, Blix said.

He said the agency also has doubled -- to once a month -- the number of visits to the reactor by its inspectors to "service our surveillance equipment, develop the films [and] determine the movements" of equipment.

Blix said the problem -- leading to the unprecedented announcement in September, 1981, that the atomic agency no longer could assure the world that no diversion was taking place from an internationally safeguarded civilian nuclear plant -- began in February, 1981, when Pakistan started inserting its own fuel into the reactor.

Until that time, Pakistan had been obtaining fuel for the Canadian-built reactor from Canada, and international inspectors thus were able to check the amount of new fuel brought into Pakistan against the amount of irradiated fuel stored in the spent-fuel pool.

"But in February, 1981," Blix said, "they started loading their reactor with homemade fuel bundles. And we couldn't verify how many fuel bundles in total their plant was manufacturing because this plant was not submitted to our safeguards.

"So in April, 1981, we informed Pakistan that, due to the fact that the whole situation had drastically changed, the safeguards at the reactor would have to be upgraded." Talks between the atomic agency and Pakistan began that month. When no agreement had been reached by September, the agency's board was told that assurances no longer could be provided.

"As of today, we are still not in a position to give such assurances," Blix said. "The installation and operation of the bundle counter and the one further item -- that is what remains."

Earlier in the day, Blix addressed the winter meeting of the American Nuclear Society and expressed the hope that the Reagan administration will soon end its two-month-old boycott of the international atomic agency and resume full participation in its activities.

The U.S. delegation walked out of the agency's annual meeting in September after members voted to reject the credentials of Israel. The administration subsequently suspended all U.S. participation in agency activities pending a "reassessment" of American membership. The United States also is withholding the final $8 million of its 1982 assessment.

State Department sources, who had indicated that the reassessment would be completed by this time, say it is expected to take another month at least.

"We have no problems at the moment with our liquidity," Blix said yesterday. "The effects of the U.S. absence lie rather in the field of cooperation."

In a move apparently designed to appeal to the longstanding U.S. interest in strengthening the agency's efforts to prevent proliferation of nuclear weapons, Blix said yesterday that he hoped to present a plan at the February board meeting "to bring up the capacity of safeguards" and provide greater monitoring of nuclear facilities around the world.