Iran claims it has begun work to further enrich its stockpile of uranium to 20 per cent, saying that the higher-grade uranium will be made into fuel for a medical research reactor in the capital Tehran.

On his part, Barack Obama, the US president, says the international community is pursuing "a significant regime of sanctions" against what Washington calls Iran's intransigence.

Obama has said the US, Russia, China, France, Britain and Germany are "moving along fairly quickly" to toughen measures against Iran.

Against this backdrop of escalating tensions, Al Jazeera's Nick Clark speaks to Rahim Mashaee, Iranian President Mahmoud Ahmadinejad's chief of staff, who says the standoff over his country's nuclear programme will continue as long as the two sides mistrust each other.