Iran 'poses major US challenge'
Iran may pose a greater challenge than any other nation to the US, Secretary of State Condoleezza Rice has said.
She said Iran was determined to develop nuclear weapons, was the "central banker for terrorism" in the Middle East and was a block to democracy.
White House spokesman Scott McClellan said later the US still wanted a diplomatic solution to the dispute.
Iran has vowed to resist international pressure, insisting it has the right to civilian nuclear technology.
It denies accusations from the US and EU that it is seeking to develop nuclear weapons.