Iran has turned away International Atomic Energy Agency inspectors from an underground site meant to shelter its uranium enrichment program from attack, diplomats and U.N. officials told The Associated Press Monday.

Adding to tensions, Iran's supreme leader deflated hopes his country would announce a readiness to freeze enrichment when it formally responds to U.N. Security Council demands it do so. On Monday, the eve of the self-imposed Iranian deadline on accepting or refusing such a moratorium, Ayatollah Ali Khamenei said that Tehran will continue to pursue the contentious nuclear technology.

Iran's unprecedented refusal to allow access to the facility at Natanz could seriously hamper international attempts to ensure Tehran is not trying to produce nuclear weapons as well as violate a key part of the Nonproliferation Treaty, the diplomats and officials told the AP. They spoke on condition of anonymity because of the sensitivity of the information.

Outlining other signs of Iranian defiance, they said that Iran denied entry visas to two IAEA inspectors in the last few weeks after doing so earlier this summer to Chris Charlier, the expert heading the agency's team to Tehran. Additionally, they said, other inspectors were only given single entry visas during their visits to the country last week, instead of the customary multiple-entry ones.

Iran's reported defiance was likely to harden Western resolve to punish Tehran if it refuses to compromise on its nuclear ambitions by agreeing to give up uranium enrichment, which can be used to create the fissile core of warheads.

IAEA head Mohamed ElBaradei is due to report by Sept. 11 to the agency's board of governors on Iran's compliance to an Aug. 31 U.N. Security Council deadline on freezing enrichment and on other aspects of Tehran's cooperation with agency attempts to establish whether it has nuclear weapons aspirations.

The council has threatened sanctions if Iran remains defiant. Diplomats on Monday told the AP these could include a ban on missile and nuclear technology to Tehran; international refusal to grant entry visas to those involved in Iran's nuclear program and a freeze of their assets as well as a ban on investment in the country.

The U.S. ambassador to the United Nations, John Bolton, said "nothing surprises me about how Iran treats its obligations" under the nuclear nonproliferation agreement. He told reporters that Iran had concealed things from inspectors in the past and alleged that Tehran also had falsified data.

Although Bolton said he had no specific knowledge of Iran's latest blocking of U.N. inspectors, he said, "More obstructionism doesn't surprise me at all."

IAEA officials refused to comment.

The Islamic republic has said it will formally respond by Tuesday to a six-power offer of economic and political rewards if it freezes enrichment and negotiates on its nuclear program. Five of the countries behind the overture — the U.S., Russia, China, Britain and France — are permanent members of the U.N. Security Council, which has set the Aug. 31 deadline on Iran's enrichment program.

But on just a day before Iran said it would formally respond, its top leader again ruled out an enrichment freeze.

"The Islamic Republic of Iran has made its own decision and in the nuclear case, God willing, with patience and power, will continue its path," Khamenei, Iran's supreme authority, was quoted as saying Monday by state television.

He accused the United States of pressuring Iran despite Tehran's assertions that it was not seeking to develop nuclear weapons, as Washington and its key allies contend.

"Arrogant powers and the U.S. are putting their utmost pressure on Iran while knowing Iran is not pursuing nuclear weapons," he said.

Iran said Sunday that it will offer a "multifaceted response" to the incentives proposal but already insisted then that a full enrichment freeze was out of the question.

Tehran says uranium enrichment does not violate any of its obligations under the Nuclear Nonproliferation Treaty, and that its nuclear program aims to produce electricity.

But U.N. officials suggested that its refusal to allow IAEA inspectors access to the underground nuclear site being built at Natanz was in itself a violation of the treaty because it contravenes Tehran's commitment under the pact to inform the Vienna-based agency of the progress of such projects.

Iranian officials have said the country intended to move toward large-scale uranium enrichment involving 3,000 interconnected centrifuges in underground halls at Natanz, in central Iran, by late this year and would later expand the program to 54,000 centrifuges.

Former U.N. nuclear inspector David Albright, president of the Washington-based Institute for Science and International Security, describes the site as a vast complex 75 feet underground, covered by layers of materials. It is unclear whether that includes concrete.

Despite optimistic forecasts by Iranian officials on the future of Natanz, however, Albright has said the Iranians have shown signs that they're having problems with the technology — a key hurdle.

Albright's group has suggested that — if it were interested in producing bombs — Iran could create a basic small plant of 1,500 centrifuges, to make enough bomb fuel for one weapon. But the group has estimated even that would take three more years.

For now, Iran's known enrichment capabilities consist of 164 centrifuges connected in series or a "cascade," at its surface pilot plant at Natanz.