Kofi Annan has been touring the region, seeking to shore up the ceasefire that halted a 34-day war between Israel and Hezbollah in Lebanon.
The UN chief's trip to Iran comes just days after Tehran failed to meet a UN Security Council deadline to halt sensitive nuclear work.
Annan held talks on Saturday about the Lebanon truce and the nuclear standoff with senior Iranian officials, including Manouchehr Mottaki, Iran's foreign minister.
After the talks, in reference to UN resolution 1701, UN spokesman Ahmad Fawzi said: "He [Mottaki] concluded by saying that we can count on his full co-operation."
Resolution 1701 drew up the terms for the ceasefire, including expanding an existing UN peacekeeping force in Lebanon and calling for an arms embargo on Hezbollah.
Iran funded and armed Hezbollah in the 1980s although it now says its support is primarily moral and political.
But Iran is still widely believed to be the main arms supplier for the Lebanese group.
Annan raised the arms embargo issue with Mottaki during his talks, Fawzi said, but did not give further details.
Fawzi said Ahmadinejad had told Annan in telephone talks prior to the visit that Iran had reservations about some articles of the resolution but had also said Iran would co-operate in its implementation.
Hamid Reza Asefi, spokesman for Iran's foreign ministry, did not say if Iran had offered to co-operate over the Lebanon resolution when he was asked about the issue at a news conference.
"We told Mr Annan about our stance. We said that it is a Lebanese resolution which needs a Lebanese solution and the Lebanese groups should reach unanimity on that," he said.
Asked if Annan had raised the arms embargo issue, he said: "Mr Annan never said that we sent any weapons. The weapons are being sent by America to Israel and that should be stopped."