VIENNA, Austria — In an unusually blunt warning, the U.N. atomic agency said Monday that its monitoring of Iran's nuclear activities is being hampered because Tehran objects to giving some agency inspectors access to its program.

The complaint by the International Atomic Energy Agency was made in a restricted report on Iran made available to The Associated Press. It follows Iran's recent decision to strip two experienced inspectors of the right to monitor Tehran's nuclear activities after the two reported undeclared nuclear experiments.

The Islamic Republic says the reporting by the two was inaccurate, but the IAEA stands by the findings.

Objections by Iran to some experienced inspectors "hampers the inspection process and thereby detracts from the Agency's capability to implement effective and efficient safeguards in Iran," the document said.

The 11-page report devoted a special section to the complaint, reflecting the importance attached to it by IAEA chief Yukiya Amano. Such a section was included in only one previous report, after Iran stripped the right of dozens of inspectors in 2006 and 2007.

The quarterly report, which was being circulated to the IAEA's 35-nation board and to the U.N. Security Council, also said Iran continues to enrich uranium in contravention of U.N. Security Council demands.

Iran insists it only wants to enrich uranium to create energy. But international concern is high because enrichment can also create the fissile core of nuclear warheads.

Low enriched uranium is used as nuclear fuel but the same process can produce weapons-grade material.

The report noted that while the rate of enrichment had not significantly changed over the past year, it was steady, with Tehran now accumulating about 2.8 tons of low-enriched material -- nearly enough for three nuclear bombs -- since its program was revealed seven years ago.

The report, by IAEA chief Yukiya Amano, also said that Iran continued to stonewall the agency in its efforts to follow up on U.S. and other intelligence indicating past experiments meant to develop a nuclear weapons program. It also warned that with the passage of time chances of establishing the accuracy of such information were diminishing.

With Iran refusing to engage on the issue for over two years, "the possible deterioration in the availability of some relevant information increase the urgency of this matter," said the report.