New York  - A commission to investigate the assassination of former Pakistani prime minister Benazir Bhutto is limited to just six months and cannot carry out criminal proceedings, UN Secretary General Ban Ki-moon said Thursday in a letter to the UN Security Council.

"The duty of determining criminal responsibility of the perpetrators of the assassination would remain with the Pakistani authorities," the letter said.

Ban said he will name three impartial people to probe Bhutto's death on December 27, 2007, during her campaign to regain leadership in the Pakistani government.

The commission's duties are to determine the "facts and circumstances" of the murder.

The commission is to receive full cooperation from the Pakistani government, freedom of movement throughout the country as well as transportation.

It has to submit a report to the Pakistani government and Ban within six months. The letter made no mention of any legal steps once the report is handed in.

This is unlike the much publicized international investigation into the assassination of former Lebanese prime minister Rafik Hariri in February 2005 in Beirut. The Security Council has approved the establishment of a tribunal at The Hague to try suspects, beginning in March. (dpa)