TEHRAN, Iran: A U.N. nuclear team arrived in Tehran early Sunday for a mission expected to focus on Iran's alleged attempt to develop nuclear weapons.

The U.N. nuclear agency delegation includes two senior weapons experts - Jacques Baute of France and Neville Whiting of South Africa - suggesting that Iran may be prepared to address some issues related to the allegations.

The delegation from the International Atomic Energy Agency is led by Deputy Director General Herman Nackaerts, who is in charge of the Iran nuclear file. Also on the team is Rafael Grossi, IAEA chief Yukiya Amano's right-hand man.

In unusually blunt comments ahead of his arrival in Tehran, Nackaerts urged Iran to work with his mission on probing the allegations about Iran's alleged attempts to develop nuclear weapons, reflecting the importance the IAEA is attaching to the issue.

Tehran has refused to discuss the alleged weapons experiments for three years, saying they are based on "fabricated documents" provided by a "few arrogant countries" - a phrase authorities in Iran often use to refer to the United States and its allies.

Ahead of his departure, Nackaerts told reporters at Vienna airport he hopes Iran "will engage with us on all concerns."

"So we're looking forward to the start of a dialogue," he said: "A dialogue that is overdue since very long."

In a sign of the difficulties the team faces and the tensions that surround Iran's disputed nuclear program, a dozen Iranian hard-liners carrying photos of slain nuclear expert Mostafa Ahmadi Roshan were waiting at Tehran's Imam Khomeini airport early Sunday to challenge the team upon arrival.

That prompted security officials to whisk the IAEA team away from the tarmac to avoid any confrontation with the hard-liners.

Iran's official IRNA news agency confirmed the team's arrival and said the IAEA experts are likely to visit the underground Fordo uranium enrichment site near the holy city of Qom, 80 miles (130 kilometers) south of the capital, Tehran.

During their three-day visit, the IAEA team will be looking for permission to talk to key Iranian scientists suspected of working on a weapons program, inspect documents related to such suspected work and secure commitments from Iranian authorities to allow future visits to sites linked to such allegations. But even a decision to enter a discussion over the allegations would be a major departure from Iran's frequent simple refusal to talk about them.

The United States and its allies want Iran to halt its enrichment of uranium, which they worry could eventually lead to weapons-grade material and the production of nuclear weapons. Iran says its program is for peaceful purposes, such as generating electricity and producing medical radioisotopes to treat cancer patients.

Iran has accused the IAEA in the past of security leaks that expose its scientists and their families to the threat of assassination by the U.S. and Israel.

Iranian state media say Roshan, a chemistry expert and director of the Natanz uranium enrichment facility in central Iran, was interviewed by IAEA inspectors before being killed in a brazen bomb attack in Tehran earlier this month.

Iranian media have urged the government to be vigil, saying some IAEA inspectors are "spies," reflecting the deep suspicion many in Iran have for the U.N. experts sent to inspect Iran's nuclear sites.