UN chief Ban Ki-moon was meeting Iran's top leaders in Tehran on Wednesday on a visit hailed by the Islamic republic as a diplomatic coup over arch-foes United States and Israel.
Ban, who was to go on to attend a summit of Non-Aligned Movement (NAM) states on Thursday and Friday in Tehran, was said to be determined to use his trip to call for Iran to take "urgent" action over its disputed nuclear drive, its human rights record and the conflict in its ally Syria.
"Iran has a crucially important role in the region, especially when it comes to Syria. I am going to discuss this with (Iran's) supreme leader," he told reporters on his arrival.
Ban was to meet Iran's supreme leader Ayatollah Ali Khamenei, President Mahmoud Ahmadinejad and Iran's chief nuclear negotiator Saeed Jalili later Wednesday, Iranian media said.
He began his visit by briefly seeing Iran's parliamentary speaker, Ali Larijani.
Although the UN secretary general is a regular attendee at NAM summits -- which gather 120 developing nations accounting for nearly two-thirds of the UN member states -- both the United States and Israel criticised his presence in Tehran.
The US State Department said such a visit "sends a very strange signal with regard to support for the international order," stressing that Iran was "in violation of so many of its international obligations and posing a threat to neighbours."
Israeli Prime Minister Benjamin Netanyahu earlier this month told Ban he would be making "a big mistake" if he attended.
Iran has seized on Ban's presence as a victory over its enemies and a sign it was not so internationally isolated as the United States has portrayed.
Overeager Iranian government officials and state media jumped the gun by saying Ban had arrived in Tehran more than two hours earlier than he actually did.
State television showed Ban being greeted effusively by Iranian officials waiting for him on the red carpet leading up to his UN plane.
Iran is engaged in a deepening showdown with the United States and the rest of the UN Security Council over its disputed nuclear programme. It has also been threatened with possible air strikes on its nuclear facilities by Israel.
The West fears the programme is aimed at developing a nuclear weapons break-out capability. Iran denies that, saying its atomic activities are exclusively peaceful.
The UN's nuclear watchdog, the International Atomic Energy Agency, is expected to release its latest report on Iran's nuclear programme this week -- perhaps concurrently with the NAM summit.
The report is likely to highlight Iran's expanding uranium enrichment activities -- which the UN Security Council has repeatedly demanded be suspended -- as well as Tehran's refusal to allow IAEA inspectors into a military site, Parchin, suspected of hosting explosives tests for nuclear warhead designs.