UNITED NATIONS, Dec 31 (Reuters) - A U.N. inquiry into the 2007 assassination of former Pakistani Prime Minister Benazir Bhutto will be prolonged by three months because of the scale of the task, the United Nations said on Thursday.

A three-member U.N. commission had been due to report by Dec. 31 following a six-month investigation.

"Because of the substantial amount of information collected by the commission in Pakistan and further follow-up work that remains, the commissioners requested additional time to complete their report," a U.N. statement said.

U.N. Secretary-General Ban Ki-moon has informed Pakistan's government and the Security Council that the inquiry will be extended until March 31, 2010, it said.

The commission, headed by Chile's U.N. Ambassador Heraldo Munoz, is looking into the circumstances surrounding the attack that killed Bhutto after an election campaign rally in Rawalpindi city on Dec. 27, 2007.

The panel is not expected to name suspected culprits. Any criminal investigation will be up to Pakistani authorities, but Munoz has said the commission's findings may be able to complement the government's efforts.

Ban set up the commission at the request of the coalition government, led by Bhutto's Pakistan People's Party.

The previous government, headed by allies of former president and army chief Pervez Musharraf, blamed then Pakistani Taliban leader and al Qaeda ally Baitullah Mehsud for Bhutto's murder. Mehsud was killed in a U.S. drone strike in August.

(Reporting by Basil Katz; Editing by Steve Gutterman)

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