The visit comes two days after Iran failed to meet a UN deadline for suspending its enrichment of uranium, paving the way to possible sanctions against the Islamic republic.

In a report on Thursday, the UN nuclear watchdog said Tehran had not halted uranium enrichment and three years of probing had been unable to confirm "the peaceful nature of Iran's nuclear programme" because of lack of cooperation.

IRNA said Iran's nuclear programme and the situation in Lebanon would be top of the agenda during Annan's two-day visit to Tehran, which is a major backer of the Hezbollah guerrilla group.

Annan is expected to seek Tehran's support for the UN resolution that halted Israeli-Hezbollah fighting on 14 August and called for a beefed-up UN force of 15,000 troops to enforce the peace. Syria, the other main ally of Hezbollah, promised on Friday to increase border patrols and work with Lebanese troops to thwart the arms flow to Hezbollah during a visit by Annan, but Israel was sceptical.

In Tehran, Annan will meet top officials including president Mahmoud Ahmadinejad, Iran's top nuclear negotiator Ali Larijani and foreign minister Manouchehr Mottaki. Hours ahead of Annan's arrival, Ahmadinejad vowed his country would pursue its disputed nuclear programme despite US pressure.