The United Nations has formally begun a six-month investigation into the killing of Benazir Bhutto, the former Pakistani prime minister.

A three-member UN commission, led by Heraldo Munoz, Chile's UN ambassador, began its work on Wednesday, Ishrat Rizvi, UN spokeswoman in Pakistan, said.

There has been little progress in the domestic investigation. Bhutto was killed in late 2007 as she campaigned to return her political party to power in parliamentary elections.

Her husband, Asif Ali Zardari, took over the party and was elected president by MPs in September 2008.

CIA officials and the Pakistani government at the time of the killing, led by President Pervez Musharraf, blamed Baitullah Mehsud, a Pakistani Taliban commander with alleged links to al-Qaida.

Truth hunt

Bhutto's party repeatedly hinted that Musharraf or his allies were involved and demanded a UN inquiry, claiming it was the only way the whole truth would be revealed.

Along with Munoz, the other UN investigators are Marzuki Darusman, a former Indonesian attorney-general, and Peter Fitzgerald, Ireland's former deputy police commissioner, who headed the initial UN inquiry into the assassination of Rafiq al-Hariri, a former Lebanese prime minister, in 2005.

Since his election, Zardari has continued lobbying for a UN inquiry, but has angered some critics within his party by apparently not urging the police to aggressively investigate the attack, seemingly content to rely on the world body.

Zardari said earlier this year that he believed the commission would "expose the financiers, the organisers, the sponsors and the conspirators of this terrorist act and bring them to justice".

Bhutto died in a gun-and-suicide bomb attack by at least one assassin in the city of Rawalpindi, close to the capital Islamabad.

There was no autopsy performed on her body and authorities hosed down the street after the attack, drawing criticism from her relatives who said it was an attempt to wash away evidence.