WASHINGTON — The Obama administration and its European allies plan to open new negotiations with Iran by demanding the immediate closing and ultimate dismantling of a recently completed nuclear facility deep under a mountain, according to American and European diplomats.

They are also calling for a halt in the production of uranium fuel that is considered just a few steps from bomb grade, and the shipment of existing stockpiles of that fuel out of the country, the diplomats said.

That negotiating position will be the opening move in what President Obama has called Iran’s “last chance” to resolve its nuclear confrontation with the United Nations and the West diplomatically. The hard-line approach would require the country’s military leadership to give up the Fordo enrichment plant outside the holy city of Qum, and with it a huge investment in the one facility that is most hardened against airstrikes.

While it is unclear whether the allies would accept anything less than closing and disassembling Fordo, government and outside experts say the terms may be especially difficult for Iran’s leaders to accept when they need to appear strong in the face of political infighting.