Irans secret nuclear facility to enrich uranium is a serious challenge to the global non-proliferation regime, and continues a disturbing pattern of Iranian evasion, Obama said in his weekly audio and video address.
That is why international negotiations with Iran scheduled for October 1 now take on added urgency, Obama said, referring to an upcoming meeting between key world powers and Iranian negotiators in Geneva.
The US President said his offer to talk with the Islamic Republic on its controversial nuclear programme remains open. My offer of a serious, meaningful dialogue to resolve this issue remains open. But Iran must now cooperate fully with the International Atomic Energy Agency, and take action to demonstrate its peaceful intentions, Obama said.
On this issue the international community is more united than ever before, Obama said.
Irans leaders must now choose - they can live up to their responsibilities and achieve integration with the community of nations. Or they will face increased pressure and isolation, and deny opportunity to their own people, he said.
He said the discovery of the secret nuclear plant showed a disturbing pattern of evasion by Tehran which added urgency to its Oct 1 talks with world powers.
Talking to CNN, US Defence Secretary Robert Gates said any possible military action against Iran would only buy time and delay Tehrans nuclear programme by about one to three years.
The reality is there is no military option that does anything more than buy time, Gates told the channel.
The estimates are one to three years or so, he said when asked about the impact of possible military options on Irans disputed nuclear sites.
The Pentagon chief meanwhile told ABC televisions This Week programme that the Iranians have the intention of having nuclear weapons, but stressed that whether Tehran had formally decided to develop them is in doubt.
Iranian President Mahmoud Ahmadinejad earlier said Obama should apologise for accusing Tehran of violating international law.
Not a chance, said an unequivocal Gates.
He said the revelation by Tehran did not necessarily challenge a 2007 National Intelligence Estimate that found Iran had halted its nuclear weapons development programme in 2003.
The US defence chief told CNN that while the United States would not rule out the use of force, there was still time for diplomacy and sanctions to persuade Iran to give up uranium enrichment work that Washington believes is designed to develop nuclear weapons.
In an interview with BBC radio, British Foreign Secretary David Miliband said Britain was 100 per cent focused on a diplomatic resolution to the spat over Irans nuclear programme, but refused to rule out military action.
We are 100 percent focused on a diplomatic resolution of this question, Miliband said.
In Pittsburgh, Pennsylvania, Russias President Dmitry Medvedev signalled that Moscow might support sanctions against Iran, after fresh revelations.
Russia, he said, will now go to six-power talks with Iran on October 1 expecting the Islamic regime to provide proof that its programme has peaceful intent, and has not ruled out eventually supporting measures against it.
The construction of this new plant was unexpected for all the countries. That was secret construction and thats the gravest thing in this situation, Medvedev told reporters after the G20 summit in the US city of Pittsburgh.