ISLAMABAD, Pakistan — The husband of slain opposition leader Benazir Bhutto on Saturday accused elements within Pakistan's government of responsibility for her murder and urged Britain and the United States to support a U.N. investigation into the killing.

"An investigation conducted by the government of Pakistan will have no credibility, in my country or anywhere else," Asif Ali Zardari said in a commentary published in the Washington Post. "One does not put the fox in charge of the henhouse." Zardari now heads Bhutto's political party, which intends to contest elections next month.

Bhutto's killing in a gun and suicide bomb attack on Dec. 27 thrust already volatile Pakistan into deep political crisis at a time of rising attacks by al-Qaida and Taliban extremists. It also added to political pressures on President Pervez Musharraf, a key U.S. ally in the war against terrorism.

Zardari's remarks were the latest in a steady stream of allegations by Musharraf's political opponents that elements within Pakistan's government were responsible for Bhutto's death, either directly or by failing to provide decent security for her as she campaigned.

Musharraf has blamed Islamic extremists and denies any suggestion of government involvement.

Controversy surrounding the cause of death also has added to suspicions. The government claimed the force of the suicide blast caused Bhutto's head to strike a metal lever on the sunroof of her SUV. Her party says she died from gunshots fired from just a few yards away before the blast — an account seemingly supported by video footage.

To fend off the criticism, Musharraf invited a team of anti-terror officers from Britain's Scotland Yard to offer technical and forensic expertise to the investigation.

Zardari reiterated earlier demands that a U.N. probe like the one investigating the 2005 assassination of former Lebanese Prime Minister Rafik Hariri was the only way to reveal the truth about the murder.