The International Atomic Energy Agency (IAEA) has been investigating since February 2003 whether Iran's nuclear programme is peaceful, or devoted to secretly developing atomic weapons, as the United States alleges.
The IAEA is to report its findings at a meeting in Vienna in June that the agency's chief Mohamed ElBaradei has said will be "key in the... consideration of Iran's implementation" of the nuclear Non-Proliferation Treaty (NPT).
An IAEA ruling that Iran is in non-compliance with the NPT would send the issue to the UN Security Council, which could then impose punishing sanctions on the Islamic Republic.
Iran had called a halt earlier this month to the inspection mission after the IAEA condemned the Islamic Republic for failing to report that it had designs for sophisticated P2 centrifuges for enriching uranium to levels that could be weapon-grade.
Tehran yielded and allowed the visit after a delay of two weeks however, due to an international outcry against Iran for failing to cooperate with the atomic agency.
The two-man IAEA team, which arrived early Saturday in Tehran after flying from IAEA headquarters in Vienna, was already on its way later Saturday to the Natanz uranium enrichment plant, 250 kilometres (150 miles) south of Tehran.
The inspectors will also visit the Isfahan nuclear technology centre, in what is a regular inspection of NPT monitoring safeguards, spokeswoman Melissa Fleming said.
A diplomat said the IAEA would not be verifying on this trip Iran's suspension of uranium enrichment.
Iran had promised in February to halt not just enriching uranium but all related activities, such as building centrifuges, in a move IAEA director general ElBaradei said was crucial to building international confidence that Iran is cooperating.
The diplomat said another inspection team slated to go into Iran in about two weeks would be "determining if the commitment to suspension is being honored."
Even more aggressive inspections are expected later in April, particularly to look into the P2 question, diplomats said.
"It seems unlikely they didn't go beyond the designs to try to make the centrifuges," one diplomat said, adding that signs of this would be in manufacturing parts needed for the gas centrifuges, or if the IAEA found traces of radiation or uranium particles from tests of the machines.
The diplomat said the IAEA also wants to be sure Iran did not obtain weapons designs, as did Libya from an international black market in nuclear technology that also supplied Iran.
The IAEA has also asked Pakistan to allow it to take environmental swipes from centrifuges there to determine whether machines Iran claims it imported from Pakistan through the black market were the source of traces of highly enriched uranium found by the IAEA in Iran.
ElBaradei is to visit Iran in early April to urge Iran to cooperate fully in answering questions about its nuclear program, Fleming said.
It will be Elbaradei's third visit to Iran since Feburary 2003.
Fleming refused to give an exact date for the visit.
She said that "with the IAEA board of governors' members calling for a comprehensive report for the next board meeting in June, Dr. ElBaradei wants to personally emphasize to the Iranians how crucial it is they act in a fully transparent and cooperative manner so that the open questions about Iran's nuclear activities can be answered."