Diplomats say Iran has assembled hundreds of advanced machines in an attempt to speed up a process in its nuclear program that can produce both fuel for power plants and the fissile core of nuclear warheads.

The diplomats say more than 300 of the centrifuges have been linked up in two separate units and a third unit is being worked on. They spoke on condition of anonymity because their information is confidential.

But they say the work is preliminary, with none of the machines running and none installed in Iran's main underground uranium enrichment plant.

Two of the diplomats talked to The Associated Press earlier this week and a third on Thursday, fleshing out details divulged by the other two.