Israel calls for action on Iran
Israel says the disclosure that Iran is building a second nuclear enrichment facility proves it "wants to equip itself with nuclear weapons".
Foreign Minister Avigdor Lieberman said Israel wanted to see an "unequivocal" Western response to the development.
Tehran insists that the site, under construction near the city of Qom, is being built in line with United Nations regulations, though this is contested.
Iran says it wants atomic power only for the production of electricity.
But the new revelations have raised tension days before talks between Iran and six global powers negotiating over Tehran's atomic programme.
Meanwhile Iranian media reported that the elite Revolutionary Guards would start missile defence exercises on Sunday, in a move which seems guaranteed to increase tensions further.
"The revelations of this second nuclear enrichment site... prove beyond any doubt that this country wants to equip itself with nuclear weapons," Mr Lieberman told Israeli public radio.
IRAN'S NUCLEAR SITES
Iran insists nuclear facilities are for energy, not military purposes
Bushehr: Nuclear power plant
Isfahan: Uranium conversion plant
Natanz: Uranium enrichment plant, 4,592 working centrifuges, with 3,716 more installed
Qom: Second enrichment plant, not yet operational
Arak: Heavy water plant
"We hope that an unequivocal answer is given on 1 October," he added, referring to the forthcoming talks.
"We are not surprised... because we have been saying that Iran is developing its nuclear activities for military purposes, and the facts prove it now.
The BBC's Paul Wood says Israel is now looking for tough international sanctions against Iran and hoping that Russia and China will be brought on board by news of the second facility.
In his weekly address on Saturday, US President Barack Obama said the discovery continued a disturbing pattern of evasion by Iran.
"My offer of a serious, meaningful dialogue to resolve this issue remains open," he said. "But Iran must now co-operate fully with the International Atomic Energy Agency (IAEA), and take action to demonstrate its peaceful intentions."
On Friday, President Obama, French President Nicolas Sarkozy and British Prime Minister Gordon Brown demanded that Iran allow UN inspectors into the second site.
In response, Iran's President Mahmoud Ahmadinejad said he had no problem with the plant being inspected, and denied it was "secret".
"According to IAEA rules, countries must inform the agency six months ahead of the gas injection in their uranium enrichment plants," he said. "We have done it 18 months ahead and this should be appreciated not condemned."
But the BBC's Paul Reynolds says there is a dispute about the amount of notice that Iran is required to give the IAEA before a new nuclear facility becomes operational.
In 2003, Iran agreed on what is called a Subsidiary Arrangement, under which it is required to tell the IAEA at the preliminary design stage. Iran later announced that it had repudiated this agreement, but the IAEA says that no such unilateral repudiation is allowed.
On Saturday, the chief of staff for Iranian Supreme Leader Ayatollah Ali Khamenei said the second enrichment plant "will become operational soon".
"God willing this new plant will become operational soon and make the enemy blind," Mohammad Mohammadi Golpayegani was quoted as saying by the semi-official Fars news agency.
Also speaking on Saturday, UK Foreign Secretary David Miliband told the BBC that the UK was "100% committed" to finding a diplomatic solution to the Iran issue, but refused to rule out military action altogether.
Israel, widely held to be the only nuclear power in the Middle East, has also refused to rule out the possibility of attacking Iran's nuclear facilities.
Israel has never officially admitted having nuclear weapons, but is widely recognised to possess at least 75 warheads.