Iran has started the process of enriching uranium to 20 per cent in defiance of the West. Iranian state media said the process has begun at the Natanz plant in the presence of International Atomic Energy Agency (IAEA) inspectors.The move came after Western powers stepped up pressure for international sanctions against Iran. The US and its western allies said Iran is trying to develop a nuclear weapon, a charge Iran has denied. The Britain has described Iran's new position as deeply worrying.Iran currently enriches uranium to a level of 3.5% but requires 20% enriched uranium for its Tehran research reactor, which is meant to produce medical isotopes. A bomb would require uranium enriched to at least 90%. Enriching to 20% would take about one year, using the 2,000 centrifuges at the underground Natanz facility, experts say.The UN Security Council has already imposed three rounds of sanctions against Iran in an effort to make it cease all uranium enrichment activities and heavy-water projects, which could produce weapons-grade plutonium.