Iran has reached another milestone in its nuclear program, U.N. inspectors reported Friday, saying the country has completed work on an underground factory for making enriched uranium. Western officials fear that the heavily fortified facility could some day be used to make a nuclear bomb.

The report by the International Atomic Energy Agency also documented a sharp rise in Iran’s stockpile of a more purified form of enriched uranium that experts say can be converted relatively easily into fissile material for nuclear weapons.

The advances are likely to fuel anxiety over Iran’s nuclear ambitions because both potentially shorten the country’s pathway to becoming a nuclear-weapons state. Iran insists it is interested in nuclear energy only for peaceful purposes.

IAEA officials chided Iran in the report for failing to take steps to ease international concerns about its nuclear program. The U.N. agency noted that Tehran still refuses to allow inspections of military facilities where the government is believed to have conducted nuclear-weapons research in the past.

“Iran is not providing the necessary cooperation,” the nuclear watchdog said in its report, prepared for an upcoming meeting of the IAEA’s 35-nation board of governors. Without further cooperation, it said, the agency “is unable to provide credible assurance . . . to conclude that all nuclear material in Iran is in peaceful activities.”

A key focus of the report was Iran’s Fordow uranium-enrichment plant near Qom, about 100 miles southwest of Tehran. In recent months, Iran’s engineers have made rapid progress at the formerly secret plant, installing thousands of fast-spinning gas centrifuge machines used to make enriched uranium. The plant was constructed inside a mountain to shield it from airstrikes, and Israeli officials have warned that the plant’s completion will make it harder for Western nations to stop Iran if it decides to build nuclear bombs.

Inspectors who visited the plant this month reported that all of the plant’s nearly 2,800 centrifuges have been installed. For unknown reasons, Iran has not begun feeding uranium hexafluoride gas into more than half of the machines, the report stated.

“It is not clear why Iran hasn’t started rolling with these new machines, but they haven’t,” said a European diplomat briefed on the IAEA’s findings. The official insisted on anonymity in discussing details of the briefings.

Despite the delay in starting up the new machines, Iran appeared to be making steady progress in expanding its stockpile of uranium enriched to 20 percent, a more concentrated product that can be readily converted to the highly enriched form used in nuclear bombs. In the past three months, Iran’s supply of this type of uranium increased by nearly half, U.N. inspectors found.

In the past, Iran has sought to ease concerns about its stockpile by converting some of its 20 percent uranium into a metal form that can’t easily be turned into weapons-grade material. For now, at least, that practice seems to have been halted, the IAEA found.

Iran claims it intends to use the 20 percent enriched uranium to make fuel for a medical research reactor. But nuclear experts say Iran already possesses far more of this type of uranium than it needs for that purpose.

The evidence of Iran’s nuclear progress could intensify demands by Congress for further economic sanctions against Tehran. So far, Iran has refused to accept curbs on its nuclear program, despite unprecedented pressure on its economy from previous rounds of sanctions and a European oil embargo.