Iran's President Mahmoud Ahmadinejad has asked the country's nuclear chief to begin enriching uranium to 20%.

伊朗总统内贾德要求国家原子能主管开始进行20%的铀浓缩工作。

Iran's nuclear programme has alarmed Western powers

The move comes amid a worsening stand-off over a Western offer for Iran to swap¹（交换） enriched uranium for nuclear fuel.

Western nations fear Iran is trying to develop nuclear weapons and have threatened new sanctions. Iran insists its programme is peaceful.

The US defence secretary urged the world to "stand together", saying there was still time for sanctions to work.

"Pressures that are focused on the government of Iran, as opposed to the people of Iran, potentially have greater opportunity to achieve the objective," Robert Gates said during a visit to Italy.

In London, the Foreign Office said Mr Ahmadinejad's announcement was "clearly a matter of serious concern".

"This would be a deliberate（故意的） breach²（违背，违反） of five UNSCRs [United Nations Security Council Resolutions]," it said in a statement.

In January, diplomats³ said Iran had informed the International Atomic Energy Agency (IAEA) that it did not accept the terms of the deal agreed in October by Iran, the IAEA and the P5+1 - the US, Russia, China, UK and France plus Germany.

Earlier this week, the US, Britain and France circulated a discussion paper on further possible sanctions against Tehran.

But China says the P5+1 must remain patient and keep pursuing a diplomatic solution to the issue.

Red line

Mr Ahmadinejad made the announcement on Iranian state television - two days after his foreign minister said a deal on swapping⁴ enriched uranium for nuclear fuel was close - a claim greeted with scepticism（怀疑） by Western powers.

"I had said let us give them [Western powers] two to three months, and if they don't agree, we would start ourselves," Mr Ahmadinejad said in a speech broadcast live.

"Now Dr [Ali Akbar] Salehi, start to make the 20% with the centrifuges（离心机） ," the president said, addressing Iran's nuclear chief who was sitting in the audience at a laser technology plant in Tehran.

Civilian⁵ nuclear power requires uranium enriched to about 3%. Weapons grade uranium needs to be enriched to 90%.

The BBC Tehran correspondent Jon Leyne, reporting from London, says Mr Ahmadinejad's announcement crosses a significant red line.

Iran says it wants to supply a research reactor⁶ with highly enriched uranium following the breakdown⁷ of the international deal to provide fuel for it.

But some Western analysts⁸ say Iran does not possess the technical know-how⁹ to make fuel rods（燃料棒） for the reactor, our correspondent says, and Western countries fear this could be a stepping stone（垫脚石，跳板） towards the manufacture of weapons-grade material.

At the very least, this is a provocative¹⁰（刺激的，挑拨的） act which will make negotiations¹¹ more difficult, our correspondent says.

Existing UN sanctions are meant to prevent the flow of any items or technology which might aid Iran in enriching uranium or developing nuclear weapon delivery systems.

The sanctions range from actual sales（实际销售） or supplies to dealings with named individuals.