That will make it much less far-reaching than an existing probe by the world body of the killing of former Lebanese Prime Minister Rafik al-Hariri, which is expected to lead to a UN-organized trial in The Hague.

UN Secretary-General Ban Ki-moon announced the Bhutto inquiry on Wednesday during a visit to Islamabad. Bhutto was killed in a suicide attack in December 2007.

In a letter to the Security Council, Ban said that after discussions with Pakistan's government and council members it had been agreed that an international commission would simply seek to determine the facts and circumstances of the killing.

The commission's mandate would be for a maximum of six months and "would not extend to carrying out a criminal investigation," Ban said. It would be up to Pakistan to determine criminal responsibility, he added.

The commission will consist of three "eminent personalities," whom Ban is expected to name shortly. It will start work once funding has been arranged, Ban said.

Terms of reference attached to Ban's letter stipulate that Pakistan will ensure freedom of movement for the commission throughout the country and provide protection for people who supply it with information.

Earlier investigations by Pakistan's previous government and the U.S. Central Intelligence Agency accused an al Qaeda-linked militant of killing Bhutto, a staunch supporter of the US-led campaign against Islamist militancy.

Some of Bhutto's aides expressed dissatisfaction over those investigations.