The UN Secretary General Kofi Annan is meeting Iran's top leaders on the second day of his visit to Tehran.
Iran is seen as a rising regional power
Talks with Supreme Leader Ali Khamenei and President Mahmoud Ahmadinejad were expected to cover the ceasefire in Lebanon and Iran's nuclear programme.
Mr Annan met other Iranian figures on Saturday. His spokesman said he was assured that Iran backed the truce.
Iran's nuclear negotiator said Mr Annan had told him he agreed talks were the best way forward on the nuclear issue.
Mr Annan's visit comes days after Iran defied a UN deadline to suspend uranium enrichment - which Iran insists is being done solely for civilian nuclear power generation, while Western powers fear a weapons programme.
Discussions are taking place in the EU and between UN Security Council members over how to respond, although there are divisions over whether to pursue punitive measures or further talks.
Mr Annan is on an extensive tour of the Middle East.
Mr Annan's visit to Tehran also aims to secure Iranian backing for UN resolution 1701, the basis of the ceasefire which ended the recent 34-day conflict between Israel and the Lebanese militant group Hezbollah.
Iran is a key backer of Hezbollah, and is widely suspected to have supplied arms used against Israel.
Resolution 1701 demands that other countries withhold supplies of arms to any group other than the Lebanese government.
Foreign Minister Manouchehr Mottaki offered Mr Annan Iran's "full support" for the peace deal, said UN spokesman Ahmad Fawzi.
He later told the Associated Press the two men discussed "full implementation" of the resolution, including the section on the arms embargo.
But Mr Mottaki also warned that any attempt to change the mission of the expanded UN force in Lebanon "would create tension", according to the semi-official Iranian news agency Mehr.
On the nuclear issue, Mr Annan and Mr Larijani both emerged from meetings saying discussions had been positive.
Annan's talks with nuclear negotiator Ali Larijani were positive
Ali Larijani told Iranian TV Mr Annan's viewpoint on the resolution of the nuclear problem was "positive".
"We support his efforts," he said. "The two sides agreed that the best way is for the problem to be solved through negotiations."
Mr Annan told Iranian reporters the talks had been "good and constructive", and UN spokesman Mr Fawzi later confirmed to AP that Mr Annan "believes [the nuclear issue]... should be resolved by negotiations".
Meanwhile, EU foreign ministers meeting in Finland agreed to give Iran more time to show willingness to co-operate on the issue.
The meeting came 11 days after Iran submitted a 21-page response to a package of EU incentives for it to give up uranium enrichment.
Details of the document have not been made public, although Iran has called for "serious talks", despite ruling out halting its nuclear programme.
EU foreign policy chief Javier Solana is to meet Mr Larijani next week for talks he says are needed to "clarify issues" in Tehran's response.
The EU foreign ministers are to meet again in two weeks and the BBC Europe correspondent says they will expect a clearer answer from the Iranians by then.
On Friday the US said it was consulting European governments about the imposition of punitive sanctions against Iran.
But correspondents point out that the US already imposes economic and diplomatic sanctions on Iran, leaving it with little further leverage against a nation rich in gas and oil.
They say Mr Ahmadinejad will have little reason to dilute his stance as he draws domestic support from populism and defiance of the West, and the removal of Saddam Hussein from Iraq has boosted Iran's regional power.