TEHRAN — Iran recently sought to acquire tens of thousands of highly specialised magnets used in centrifuge machines, according to experts and diplomats, a sign that the country may be planning a major expansion of its nuclear program that could shorten the path to an atomic weapons capability.Purchase orders obtained by nuclear researchers show an attempt by Iranian agents to buy 100,000 of the ring-shaped magnets — which are banned from export to Iran under U.N. resolutions — from China about a year ago, those familiar with the effort said. It is unclear whether the attempt succeeded.Although Iran has frequently sought to buy banned items from foreign vendors, this case is considered unusual because of the order’s specificity and sheer size — enough magnets in theory to outfit 50,000 new centrifuges, or nearly five times the number that Iran currently operates.The revelation of the new orders for nuclear-sensitive parts coincides with Iran’s announcement that it plans to add thousands of more-advanced, second-generation centrifuges that would allow it to ramp up its production of enriched uranium even further, analysts said.Iran insists that its nuclear programme is peaceful and that its enrichment efforts are directed toward medical research and energy production. The disclosure of the purchase attempt comes at a time when the country is seeking to gain diplomatic leverage ahead of negotiations on proposed limits to its nuclear program.The attempt, nonetheless, has fueled Western concerns that Iran is planning a major expansion in its nuclear capacity that would allow it to make atomic weapons quickly if it chooses to do so.