The Iranian President, Mahmoud Ahmadinejad, has said his country's nuclear programme is unstoppable.
His comments come as the five permanent members of the UN Security Council and Germany gather in London to debate tougher sanctions against Tehran.
However, Iran's chief nuclear negotiator, Ali Larijani, stressed that Tehran was still ready for talks, but warned that Iran would act proportionately to any further pressure.
The US insists it wants a diplomatic solution, but has not ruled out military action.
