TEHRAN, Iran — U.N. inspectors entered a once-secret uranium enrichment facility with bunker-like construction and heavy military protection that raised Western suspicions about the extent and intent of Iran’s nuclear program.

The visit Sunday by the four-member International Atomic Energy Agency team, reported by state media, was the first independent look inside the planned nuclear fuel lab, a former ammunition dump burrowed into the treeless hills south of Tehran and only publicly disclosed last month. The inspectors are expected to study plant blueprints, interview workers and take soil samples before wrapping up the three-day mission.

No results from the inspection are expected until the team leaves the country, but some Iranian officials hailed the visit as an example that their nuclear program was open to international scrutiny.

“IAEA inspectors’ visit to Fordo shows that Iran’s nuclear activities are transparent and peaceful,” the Islamic Republic News Agency quoted lawmaker Hasan Ebrahimi as saying.

Another test of Iran’s cooperation is fast approaching, however. Iran has promised to respond this week to a U.N.-brokered deal to process its nuclear fuel abroad — a plan designed to ease Western fears about Iran’s potential ability to produce weapons-grade material.

The current inspection of Iran’s second enrichment site came about a month after Tehran disclosed its existence in a letter to the IAEA, the U.N.’s nuclear watchdog. The notification to the U.N. agency came just days before President Barack Obama and other Western leaders claimed Iran has been hiding the facility from the world for years.

After Iran’s disclosure, U.N. Secretary-General Ban Ki-moon told Iranian President Mahmoud Ahmadinejad that “the burden of proof is on Iran” to convince the international community its nuclear program is peaceful.

Iran said that by reporting the existence of the site voluntarily, it “pre-empted a conspiracy” by the United States and its allies who were hoping to present the site as evidence that Iran was developing its nuclear program in secret.

But the IAEA says Tehran should have reported it before it started construction. And the new facility, about 20 miles north of the holy city of Qom, immediately raised suspicions about the aim of the nuclear
program — which Iran claims is only for peaceful research and energy production. The site is reached by tunnels and is protected by military installations including missile silos and anti-aircraft batteries, Iranian officials said last month.

Iran says the facility — known as Fordo after a village believed to have the largest percentage of fighters killed in the 1980-88 war with Iraq — was fortified to protect against any possible attack by the U.S. or Israel.

Officials say the plant won’t be operational for another 18 months and would produce uranium enrichment levels up to 5 percent, suitable only for peaceful purposes. Weapons-grade material is more than 90 percent enriched.

Iran says its other known enrichment facility — a much larger industrial-scale plant in Natanz in central Iran — is also only to produce nuclear fuel and not at levels for weapons. But many experts say the enrichment centrifuges could be expanded and upgraded to make weapons-grade material.