WASHINGTON (Reuters) - The United States and other Western nations plan to demand that Iran immediately close and ultimately dismantle a recently completed nuclear facility deep under a mountain as part of new talks with Tehran over its nuclear program, the New York Times reported on Saturday.

The Obama administration and its European allies also will call for a halt in the production of higher-level enrichment of uranium fuel, and the shipment of existing stockpiles of that fuel out of Iran, the newspaper said, citing U.S. and European diplomats.

The diplomats told the Times that they could not imagine any agreement that left Iran with a stockpile of fuel, enriched to 20 percent purity, that could be converted to the grade needed to make an atomic bomb in a matter of months.

"We have no idea how the Iranians will react," a senior Obama administration official told the newspaper. "We probably won't know after the first meeting."

The opening talks are tentatively set for Friday.

In January, major powers signaled willingness to reopen the talks about curbing Iran's suspected pursuit of nuclear weapons but said Tehran must show it was serious about negotiations. Iran says its nuclear program is aimed solely at generating power.

The United States, Britain, France, Russia, China and Germany are the six powers involved in diplomacy aimed at resolving the long-running row over Iran's atomic plans.

Russia and China recently joined the four Western powers in expressing "regret" over Iran's expansion of higher-grade enrichment, most of which is now taking place deep inside a mountain near the Shi'ite Muslim holy city of Qom to better protect it against Israeli or U.S. attacks.

The focus on diplomacy followed rising tensions between the West, which is seeking to cut Iran's oil sales, and Tehran, which threatened to close the Strait of Hormuz through which almost one-fifth of oil traded worldwide flows.

The United States has gradually tightened sanctions on Tehran due to its failure to answer questions about its nuclear program.

(Writing by Paul Simao; Editing by Nick Macfie)