TEHRAN: Iran on Wednesday spurned a US offer to supply it with medical isotopes if it stops further enriching uranium as world powers warned the time for diplomacy was limited and the Obama administration slapped new sanctions on several affiliates of Iran’s Revolutionary Guard Corps.

The Foreign Ministry dismissed the US offer as “not logical,” after State Department spokesman Philip Crowley floated the idea on Tuesday when Iran said it had begun enriching uranium to 20 percent for a Tehran research reactor.

“Shutting down the reactor or stopping the production of medicine is not the solution,” Foreign Ministry spokesman Ramin Mehmanparast told reporters. “This proposal is not logical.”

Tehran’s announcement that it was proceeding to further enrich its low-enriched uranium (LEU) to 20 percent sent alarm bells ringing in the West, which suspects Iran’s nuclear project has military aims despite denials.

The new US sanctions targeted one person and four companies for penalties over their alleged involvement in producing and spreading weapons of mass destruction.

The Treasury Department said it is freezing the assets in US jurisdictions of Revolutionary Guard Gen. Rostam Qasemi and four subsidiaries of a previously penalized construction firm that he runs. The sanctions expand existing US unilateral penalties against elements of the Guard Corps, or IRGC, which Western intelligence officials believe is spearheading Iran’s nuclear and missile programs. The announcement came as the administration is pressing to impose fresh international sanctions on Iran over its failure to prove its nuclear program is peaceful.

Russia — the country’s long-time nuclear ally — has also toughened its stance after Iran’s move to further boost enrichment. In Moscow, Deputy Foreign Minister Sergei Ryabkov told the Interfax news agency the prospect of new sanctions against Iran was now more realistic.

“In this new situation, of course, the question of sanctions, of drafting a resolution for new sanctions has become more realistic,” he said, also adding that in Russia’s view “sanctions were not a solution to the problem.”

The major world powers of the UN Security Council permanent five plus Germany have so far followed a “dual track” approach of negotiations coupled with punitive action in a bid to secure the suspension of Iranian uranium enrichment.

“We had a dual track but it is less and less likely that we are going to be able to maintain that dual track and not move to the pressure track very soon,” Ellen Tauscher, US undersecretary of state for Arms Control and International Security, told reporters.

“We are running out of time to do what is right ... and to ease the tension in the region,” she said at NATO headquarters.