• Facility built in mountain at Qom raises suspicions • Other construction could be in progress, report says

The United Nations nuclear watchdog has expressed fears that Iran may have other secret nuclear sites following the discovery of the facility hidden in a mountain near the holy city of Qom.

The International Atomic Energy Agency, in a report published today, said the previously secret site at Fordo was in "an advanced state of construction" and was scheduled to start up in 2011.

The IAEA reprimanded Iran for failing to inform it until September about the site, even though construction had begun at least two years ago.

In a more pointed criticism of Iran than usual, the IAEA says the delay "reduces the level of confidence in the absence of other nuclear facilities under construction and gives rise to questions about whether there were any other nuclear facilities not declared to the agency".

The expression of concern comes at a sensitive moment, with no sign of a peace deal between Iran and the US, backed by Britain, France and Germany. Iran has not yet formally replied to a compromise offered by Barack Obama, who said at the weekend that time was running out.

The IAEA sent inspectors to the Fordo site late last month but today indicated it is to seek clarification on several issues, in particular how long the Fordo site had been planned.

The report said that technicians had moved sophisticated technical equipment into the uranium enrichment facility situated deep inside the mountain. An IAEA official said the facility was designed to produce about a ton of enriched uranium a year, enough for a small warhead.

The report quotes Iran insisting it "did not have any other nuclear facilities that were currently under construction or in operation that had not yet been declared to the agency".

Iran claims it is only intent on using nuclear energy to help meet its electricity needs.

The report is the last by the controversial head of the IAEA, Mohamed ElBaradei, who has been accused by the US of being too soft on Iran. In contrast with the concern expressed in the report, ElBaradei only last week, in an interview with the New York Times, played down the significance of the previously undisclosed site, saying it was "nothing to worry about".

The report may reflect the thinking of the IAEA's inspectors and ElBaradei's political staff, who have tended to be more sceptical about Iran's intentions than their chief.

The US, along with Britain, France and Germany, claims that discovery of the Fordo site puts Iran in breach of its international treaty obligations. Although Iran reported its existence to the IAEA in September, the US said this was to pre-empt an announcement exposing the site.

Iran said work on the site only began in 2007 but the US said it started in 2002-04 and, after a pause, resumed in 2006.

The IAEA reprimanded Iran, saying its "failure to notify the agency of the new facility until September 2009 was inconsistent with its obligations".

Israel has threatened military strikes against Iran's nuclear facilities to prevent it acquiring a nuclear weapons capability.

The report said it was concern about just such an attack that prompted Iran to build the facility inside the mountain, according to a letter from Iran to the IAEA on 28 October. "As a result of the augmentation of the threats of military attacks against Iran, the Islamic Republic of Iran decided to establish contingency centres for various organisations and activities," the report said.

The Federation of American Scientists, which tracks nuclear proliferation, said today: "Of course, there is the question of whether Fordo is simply the only 'secret' facility that we know about. The danger is that there are other facilities that can escape safeguards because the IAEA does not know about them."

It added: "The good news in this story is that the facility is now known and the IAEA kicked in exactly as it should."

Russia today dashed Iranian hopes that a Russian-built nuclear reactor at Iran's southern port Bushehr will be switched on this year, providing Iran with its first operating nuclear power plant.

Iranian parliamentarians reacted angrily to the Russian announcement.

Russia has long backed Iran in the nuclear stand-off, blocking tough UN security council sanctions, but Obama has been pressing Russian leaders to join the US, Britain, France and Germany in a show of solidarity on the nuclear issue.