The announcement of a second uranium enrichment site puts Iran on the defensive as it tries to head off further sanctions.
The site - said to be near Qom - was acknowledged by Iran in a letter to the International Atomic Energy Agency (IAEA) on Monday, only just pre-empting an announcement by US President Barack Obama. Western intelligence agencies discovered the site some time ago, according to the New York Times.
In its letter to the IAEA, Iran sought to downplay the site's importance, saying that it was a pilot plant still under construction.
The announcement about this site is an embarrassment to Iran, which has said that it is cooperating with the IAEA.
The problem for Iran is that this will increase the suspicions many governments have about its secrecy and its intentions. Under the terms of its agreement with the IAEA, it should have told the agency at the planning and design stage. Iran has tried to repudiate this agreement, so might argue that it did not yet have to report the plant, but the IAEA says that such repudiations are not permitted.
Some fear that Iran is developing at least a nuclear-weapon capacity, with a view to making a bomb one day. Iran says it is against nuclear weapons and is simply making fuel for nuclear power.
Iranian ambitions for this site are not known. It could be that they wanted a back-up in case their main plant at Natanz was attacked. But another fear is that they intended to enrich uranium more highly at the secret plant, to a level suitable for a nuclear explosion.
President Obama said that its size and scope was "inconsistent" with a peaceful programme.
Mark Fitzpatrick of the International Institute for International Affairs said the 3,000 centrifuges estimated to be at the plant would not be enough to make any nuclear fuel but could be used to enrich enough uranium to the higher level needed for a nuclear explosion.
He said that Iran did appear to be in violation of its IAEA safeguards agreement.
"Common sense also dictates that at this time Iran would have been open if it had nothing to hide," he added. "This shows it is far from being on the up-and-up."
The discovery will strengthen the demands by the US and its allies for further sanctions to be imposed on Iran unless it suspends all enrichment, as required by the Security Council.
This might help explain why this week Russia appeared to soften its opposition to further sanctions.
The US wants Iran's oil and gas industries to be targeted. Current Security Council sanctions aim principally at its nuclear and ballistic missile work.
President Obama demanded that the new site be opened immediately to IAEA inspection.
According to CNN...
The second nuclear facility, on a military base near the Shia Muslim holy city of Qom, is thought to be capable of housing 3,000 centrifuges, not enough to produce nuclear fuel to power a reactor, but sufficient to manufacture bomb-making material, a U.S. diplomatic source who read the letter told CNN.