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 an commentary published in The Washington Post. "One does not put the fox in charge of the hen house." 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 has also 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. On Saturday, members of the team examined the crime scene, which has was washed down soon after the killing.

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.

He urged "friends of democracy in the West, in particular the United States and Britain, to endorse the call for such independent investigation."

"Those responsible - within and outside of government - must be held accountable," wrote Zardari.

Pakistan's government does not support a U.N. probe, and Washington has already indicated it sees no need for one.

Also Saturday, the government accused a leading international think tank of "promoting sedition" for issuing a report urging Musharraf to resign.

The report by the Brussels-based International Crisis Group called on the United States to use the Pakistani military to persuade the former general to step down, saying Musharraf was "a serious liability, seen as complicit" in Bhutto's death.

In a statement, the government said the report "amounts to promoting sedition" and the group "neither has the credentials, nor the credibility and lacks representational standing specially on Pakistan's national affairs" to comment on Pakistan.

Sedition is a serious offense in Pakistan, but it was not clear whether the government intended to launch a criminal investigation into the International Crisis Group.

The group - which lists former Australian foreign minister Gareth Evans and other international figures as members of its board - was not immediately available for comment Saturday.