UN special representative to Iraq Ad Melkert's convoy was hit by a roadside bomb on Tuesday, but he escaped unhurt, police source in the southern city of Najaf said.

Melkert's convoy was on his way back to Baghdad from Najaf, 160 km south of the capital after his meeting with powerful Shitte cleric Ali al-Sistani, the source told Xinhua on condition of anonymity.

One Iraqi policeman was killed and four others were injured during the attack close to Najaf, he added.

Sistani is the current highest-ranking Shiite Muslim religious scholar in Iraq who lives in Najaf. As an important political figure in post-invasion Iraq, he usually keeps himself out of normal politics in the country. But with his great influence over Iraqi Shiites, he has urged them to vote actively in the March election for a better future of Iraq.

According to Sistani's office, Melkert after the meeting urged all Iraqi political factions to reach an agreement over the new government formation as soon as possible.

The Dutch politician Ad Melkert was chosen by United Nations Secretary-General Ban Ki-moon as his top envoy in Iraq in 2009.

Melkert heads the UN mission in Iraq, known as UNAMI, which is tasked with assisting in the reconstruction of Iraq, the implementation of the International Compact with Iraq and the process of national reconciliation and inclusive political dialogue.

This was not the first time that UN office was attacked by insurgents. A truck bomb at the UN's Baghdad headquarters killed 22 people in 2003, including then-US envoy to Iraq Sergio Vieira de Mello, a Brazilian.