Iran, which insists its nuclear programme is aimed at electricity generation rather than weapons production as feared by the West, will meet the US and five other powers in Geneva on Thursday.
Mr Obama warned Iran yesterday it would face 'sanctions that bite' if it did not come clean.
'This is a serious challenge to the global non-proliferation regime and continues a disturbing pattern of Iranian evasion,' he said today.
Britain, France and Germany have joined the US in raising the possibility of new sanctions against Iran if it does not take steps to address concerns about its nuclear programme.
Russia also signalled a greater willingness to go along with sanctions while China said it favoured a 'dual track' approach of pressure and talks.
UN Secretary General Ban Ki-moon expressed 'grave concern' in talks with Iranian President Mahmoud Ahmadinejad and said Tehran had to show its intentions were peaceful.
Facility to be 'operational soon'
Iran acknowledged it had a uranium enrichment facility near Qom for the first time on Monday in a letter to the UN's International Atomic Energy Agency.
The head of Iran's atomic energy organisation said Iran had assumed the disclosure would be welcomed.
A senior Iranian official in the office of Supreme Leader Ayatollah Ali Khamenei was quoted as saying the plant would start running 'soon'.
'This new plant, God willing, will soon become operational and will make the enemies blind,' the Fars News Agency quoted him as saying.
Iran already has a uranium enrichment plant near the central city of Natanz.
