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 official IRNA 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 says 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 (30 kilometers) 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 United States 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.

Another worry for the West is Iran's plans to install a more advanced type of centrifuge at the Fordo site, capable of enriching uranium several times faster and with higher efficiency.

Iran also has promised to respond later this week on U.N.-drafted proposal to have its nuclear fuel processed in Russia, which would limit Iran's stockpiles and allow more international controls.

Although Iran has not given its official answer on the proposed nuclear deal — discussed last week after talks in Vienna with the United States, France and Russia — there are increasing doubts that Iran's leadership will come on board.

On Saturday, Parliament Speaker Ali Larijani claimed the West was trying to "cheat" Iran under the deal that would ship most of Iran's uranium to Russia for reactor-ready enrichment.

Larijani, the country's former nuclear negotiator, said Iran prefers to buy the nuclear fuel it needs for a reactor under construction that makes medical isotopes.

He did not specifically address the fuel needs for Iran's planned full-scale reactor, but Russia is required to provide fuel as part of an agreement to build it for Iran in the southern city of Bushehr. The reactor is nearly operational.

Rejection of the U.N. deal would force the United States and its allies to either return to talks or step up demands for greater economic sanctions and seek to further isolate Iran.

The four-member delegation from the International Atomic Energy Agency is led by Herman Nackaerts, director of IAEA's division of operations department of safeguards. The inspectors are expected to stay three days in Iran.

They are expected to compare Iran's engineering plans with the actual layout of the plant, interview employees and take environmental samples to check for the presence of nuclear materials.

The small-scale site is meant to house no more than 3,000 centrifuges — much less than the estimated 8,000 machines at Natanz.

A recent satellite image provided by DigitalGlobe and GeoEye shows a well-fortified facility built into a mountain about 20 miles northeast of Qom, with ventilation shafts and a nearby surface-to-air missile site, according to defense consultancy IHS Jane's, which did the analysis of the imagery. The image was taken in September.

GlobalSecurity.org analyzed images from 2005 and January 2009 when the site was in an earlier phase of construction and believes the facility is not underground but was instead cut into a mountain. It is constructed of heavily reinforced concrete and is about the size of a football field — large enough to house 3,000 centrifuges used to refine uranium.