JERUSALEM - Western diplomats called on Iran Wednesday to stop enriching uranium to a level where it can be quickly upgraded for use in a nuclear weapon and to return to negotiations to reassure the world of the peaceful intent of its nuclear program.

Diplomats from the U.S., France, Germany and the U.K. said all 15 Security Council members expressed concern over Iran's move to enrich uranium to a higher level.

On Monday, the International Atomic Energy Agency confirmed that Iran has begun enriching uranium to 20 per cent at an underground bunker. That is a significantly higher concentration than the nation's main stockpile, which is enriched to 3.5 per cent -- sufficient to produce energy in nuclear reactors.

Britain's deputy U.N. ambassador Philip Parham brushed aside Iran's claim that the uranium was being enriched for use in a research reactor.

"This further step is a further clear breach of Security Council resolutions," he said. "The location, the size, the clandestine nature of the facility at Qom raises serious doubts about what its ultimate purposes are."

Parham added that Iran already has more than five years worth of 20 per cent enriched fuel for the research reactor "and it hasn't even installed the equipment that would necessary to manufacture the fuel elements for that reactor."

U.S. deputy ambassador Rosemary DiCarlo called on Iran to return to negotiations and said the U.S. would continue applying sanctions until Iran comes into compliance with Security Council and IAEA resolutions.

She pointed out that the U.N. passed sanctions a year ago and said Iranian President Mahmoud Ahmadinejad "has acknowledged to his own parliament that they're having an impact."

"Our goal is to work with our partners to insure that the sanctions are fully and robustly implemented," she said. "We will continue working with partners here on the council and elsewhere to that end."

Diplomats said the Security Council, in its closed-door discussions, did not touch on the subject of Mostafa Ahmadi Roshan, the Iranian nuclear scientist killed Wednesday by a bomb attached to his car by a passing bicyclist.