Ad Melkert was unhurt, the UN said, but unconfirmed reports said a policeman died and three others were injured.

Iraqi police said the bomb hit the second-to-last vehicle in the convoy as it left the Shia holy city of Najaf, 160km (100 miles) south of Baghdad.

Mr Melkert had been in Najaf to meet Grand Ayatollah Ali al-Sistani, Iraq's most revered Shia cleric, the UN said.

Mr Melkert, a Dutch politician, was appointed special representative for Iraq by UN Secretary General Ban Ki-moon in 2009.

The convoy, made up of UN and police vehicles, had been on his way to the airport. The UN said Mr Melkert later returned safely to Baghdad.

The attack comes as some Iraqis are looking to the UN to play a stronger role in breaking political deadlock in the country seven months after indecisive elections, our correspondent adds.

The UN scaled down its presence in Iraq following the bombing of its Baghdad headquarters in 2003.

The attack killed then-UN envoy Sergio Vieira de Mello and 21 staff members.