SALZBURG, Austria — Iran is poised to double its output of higher-enriched uranium at its fortified underground facility, the UN nuclear agency said Friday — a development that puts Tehran within months of being able to make the core of a nuclear warhead.

The finding also calls into question a tentative new US plan meant to induce Tehran to compromise on its nuclear program by offering a rollback of crippling new anti-Iran sanctions if Tehran cuts back on enriching uranium to 20 percent.