UN chief Kofi Annan has received assurances from Tehran that it will co-operate fully with the UN resolution on Lebanon, his spokesman says.
Mr Annan is holding an intensive round of talks
Mr Annan is in Iran for talks on strengthening the UN-sponsored truce which halted 34 days of conflict.
He also met the top nuclear official for "good" and "constructive" talks.
The visit comes two days after the UN nuclear watchdog reported that Iran had failed to meet the Security Council's deadline to halt uranium enrichment.
On arrival in Tehran, Mr Annan said he expected to discuss "issues of concern to this region and to the international community".
It is the latest stop on Mr Annan's tour of a crisis-ridden Middle East.
Mr Annan began his time in Iran by meeting Foreign Minister Manouchehr Mottaki and nuclear negotiator Ali Larijani.
Iran may accept that Lebanon now needs a period of calm to rebuild
He was expected to seek Iranian backing for UN resolution 1701, which ended the war in Lebanon.
A spokesman for Mr Annan said that when he and Mr Larijani had discussed the resolution, Mr Larijani had said: "You can count on our full support, sir."
As he met Iranian leaders, the first major new contingent of foreign troops to expand the UN force tasked with policing the ceasefire arrived in southern Lebanon.
Nearly 900 Italian soldiers have begun landing from boats in the southern port city of Tyre.
They will be part of a UN force which will eventually total up to 15,000 troops.
Resolution 1701 calls for other countries to withhold supplies of arms to any group other than the Lebanese government.
Along with Syria, Iran is a key supporter of Hezbollah.
It says its support for Hezbollah is primarily political, although analysts say it remains a major supplier of weaponry.
The BBC's diplomatic correspondent Jonathan Marcus says although Iran is unlikely to give up its strong support for Hezbollah, it may well meet Mr Annan half-way.
This could amount to tacitly accepting a period of calm during which Hezbollah can regroup and southern Lebanon's infrastructure can be rebuilt, our correspondent says.
Iran's nuclear programme also featured in talks between Mr Annan and Iranian leaders.
The Security Council is divided over how to respond to Iran
Both Iran and the UN said talks between Mr Annan and Mr Larijani were very positive, but neither side gave details.
The UN said it was an interesting and useful meeting, but they wanted to wait for talks on Sunday with Iran's Supreme leader Ali Khamenei and President Mahmoud Ahmadinejad before making a final assessment of Iran's mood.
Iran strongly asserts its right to uranium enrichment, insisting it is solely for civilian power generation.
Hours before Mr Annan's visit, the Iranian president repeated his warning that Iran would not abandon its nuclear programme.
"They (the West) should know that this nation will not give up its absolute right to benefit from nuclear energy even one iota," Mr Ahmadinejad told an applauding crowd in the city of Miandoab.
Western powers suspect Iran may have ambitions to create a nuclear bomb.
Six world powers are set to meet next week to decide on the way forward, after Iran missed the UN's 31 August deadline to halt enrichment.
The EU has indicated it is willing to give Iran extra time, during talks between the EU's foreign policy chief, Javier Solana and EU foreign ministers.
Correspondents say it is not clear what sanctions the UN could agree to impose on Iran as the key powers are deeply divided over whether punitive measures should be taken.