A UNITED NATIONS report will confirm this week that Iran is expanding its nuclear program in defiance of the international community, bolstering calls for tougher sanctions amid the threat of an Israeli military strike.

The International Atomic Energy Agency (IAEA), the UN’s nuclear watchdog, is expected to detail as early as today how Iran has installed hundreds of new centrifuges – which are used to enrich uranium – at two nuclear facilities. It is also expected to report a rise in production rates.

The agency is also likely to voice concern about the use of the Parchin military base, southeast of Tehran. Satellite images showed last week that a pink canvas had been placed over what analysts believe is ground that was used to conduct nuclear weapons-related experiments, shrouding the area from view.

IAEA inspectors allowed to visit Iran’s nuclear compounds have been denied entry to Parchin. Iran says its nuclear program is for peaceful purposes. Britain, the US Israel and their allies suspect that Iran wants to develop nuclear weapons.

“The level of anxiety regarding the Iranian threat is very high and is only becoming exacerbated by the fact that the nuclear program is moving forward,” a senior Israeli diplomat told The Times.

“Is the clock of the nuclear program moving forward faster than the clock of the sanctions and the pressure? Yes,” the diplomat said. “The sanctions aren’t tough enough and need to go further.”

Of particular concern is the expansion of the Fordow nuclear facility, which is deep underground in the mountains, making it harder for Israeli jets to destroy – a threat that Tel Aviv has levied as a last option.

The Israeli diplomat said that Iran was in the process of doubling its capacity at Fordow to about 1,500 centrifuges, increasing the amount of 20 per cent-enriched uranium it could produce. Uranium enriched to 20 per cent fuels Iran’s main research reactor, but it is also just below the level usable in nuclear bombs.

Capacity and production at Natanz, a larger but less-well protected facility, was also on the rise, according to the diplomat. Natanz is the primary site for producing uranium enriched to 3 to 5 per cent, the level required to fuel civilian-use nuclear reactors.

Mark Fitzpatrick, the director of the Nonproliferation and Disarmament Program at the International Institute for Strategic Studies, said the IAEA report would apparently show that 1,000 more centrifuges have been installed in total, but were not operational. “The report probably will show more centrifuges in place, more uranium being enriched every month and obviously more of a stockpile,” he said.

Hugh Chalmers, a research analyst at the Royal United Services Institute, had heard suggestions that the IAEA report could point to a cut in the time it would take to produce one bomb’s worth of uranium enriched to more than 90 per cent, the level at which it can be used in a nuclear weapon.

“A reduction by 50 per cent from two months to one month would certainly alarm people,” he said. “But it doesn’t imply that they will pursue weapons-grade uranium and it doesn’t answer the question of what they could do with the material once they got it.”

Mr Fitzpatrick doubted that Iran would be able to produce the raw material for a nuclear bomb within two months. “Such worst-case analysis can easily become a kind of drumbeat for war,” he said. The IAEA has set up an Iran Task Force to handle its inspections of Tehran’s atomic activities.

The move came as Ban Ki Moon, the UN Secretary-General, paid a rare visit to Iran before a summit over the next two days.