Diplomats say the U.S. is considering softening demands that Iran gut its uranium enrichment program in favor of letting Tehran keep nearly half of it intact but placing other constraints on its possible use as a path to nuclear weapons.

Ideally, the U.S. wants no more than 1,500 centrifuges enriching. Iran insists it wants to use the technology only to make reactor fuel and demands it be allowed to run at least the present 9,400 machines.

The tentative new offer envisages letting Iran keep up to 4,500 centrifuges but would cut the stock of uranium gas fed into the machines to where it would take more than a year of work to create enough material for a nuclear warhead.

The diplomats demanded anonymity Thursday because their information is confidential.