Mr Obama, who said Mr Mousavi had 'captured the imagination' of Iranians who want to open up to the West, hailed Mousavi supporters at a White House news conference.

'Their bravery in the face of brutality is a testament to their enduring pursuit of justice. The violence perpetrated against them is outrageous,' Mr Obama said.

The US president said he did not take seriously Mr Ahmadinejad's call for him to apologise for criticising Tehran, 'particularly given the fact that the United States has gone out of its way not to interfere with the election process in Iran.'

Washington had been hoping to convince Tehran to drop what it suspects are plans to develop nuclear bombs, while also seeking its cooperation in stabilising Afghanistan and Iraq. The government says its nuclear programme is to generate power.

'Iran's possession of nuclear weapons will trigger an arms race in the Middle East that would be bad ... for the security of the entire region,' said Mr Obama.

He added: 'So even as we clearly speak out in a unified voice in opposition to the violence that's taken place in Iran, we also have to be steady in recognising that the prospect of Iran with a nuclear weapon is a big problem.' -- REUTERS