U.N. nuclear inspectors have begun a three-day task that could greatly influence the direction and urgency of U.S.-led efforts to rein in Iran’s ability to enrich uranium.

The critical mission to Iran involves a probe of allegations of a secret atomic weapons program. The investigation comes against a backdrop of escalating Western economic pressures and warnings about safeguarding Gulf oil shipments from possible Iranian blockades.

Washington and allies fear Iran’s enrichment program could eventually produce weapons-grade uranium. Iran has declined to abandon its enrichment labs, but claims it only seeks to fuel reactors for energy and medical research.

The International Atomic Energy Agency team is likely to visit an underground enrichment site near the holy city of Qom, 80 miles south of Tehran. It’s carved into a mountain as protection from possible airstrikes.