Three more detectives arrived from London, including an expert in the type of explosives used in the gun and suicide bomb attack which killed Bhutto and nearly two dozen of her supporters at a rally in Rawalpindi on December 27.

The first batch of British anti-terrorism detectives arrived last week following an invitation from President Pervez Musharraf, who had admitted Pakistani investigators had made mistakes in the initial probe of the murder.

They have collected forensic evidence, reconstructed the crime scene, and recorded statements from witnesses and the medical team that tried to revive Bhutto after the attack.

"There is no breakthrough yet. We are examining the evidence closely with all technical help and expertise from Scotland Yard," a senior Pakistani investigator said.

Bhutto's brazen murder after repeated threats to her life threw the Islamic republic, a key US ally in the "war on terror," into turmoil and raised international concerns for the security of its nuclear arsenal.

The head of the UN atomic watchdog, Mohamed ElBaradei, said in statements published on Tuesday that he feared Pakistan's nuclear warheads could fall into extremist hands.

"I fear that chaos... or an extremist regime could take root in that country which has 30 to 40 warheads," ElBaradei told the pan-Arab daily Al-Hayat in an interview.

The International Atomic Energy Agency (IAEA) chief stressed that he was "worried that nuclear weapons could fall into the hands of an extremist group in Pakistan or in Afghanistan."

There has been worldwide concern over the security of Pakistan's nuclear warheads since November, when Musharraf imposed a state of emergency for six weeks to curb Islamist violence.

A foreign ministry spokesman angrily rejected ElBaradei's concerns.

"Pakistan is a responsible nuclear weapons state," spokesman Mohammad Sadiq told a weekly press briefing.

"Our nuclear weapons are as secure as any other nuclear weapons state. We therefore believe statements expressing concern about their safety and security are unwarranted and irresponsible."

US Senator Joe Lieberman, who held talks with Musharraf and military chiefs here Wednesday, said he had received a "detailed and specific" briefing about the safety of Pakistan's atomic weapons.

"Overall I felt reassured... and I will take that message back to Congress," the chairman of the US Senate's Committee on Homeland Security and Governmental Affairs told reporters.

He also said Musharraf had insisted that February 18 general elections, seen as a crucial step in restoring full civilian rule after the ex-general's 1999 military coup, would be free and fair.

"The president could not have been stronger or more explicit that that should happen and that the government do everything they can to make sure that the elections are fair and credible," Lieberman said.

The British detectives met Musharraf for the first time on Tuesday and received his assurances of the government's full support amid allegations from Bhutto's supporters that it knows more than it is saying about the murder.

Bhutto's Pakistan People's Party (PPP) has demanded an independent UN investigation and accused the government of a "conspiracy" over the slaying of its most outspoken critic.

The PPP says Musharraf failed to provide sufficient security for the two-time former prime minister after an October suicide bombing failed to kill her.

The government has blamed a local Al-Qaeda-linked militant for organising the assassination, but he has denied any involvement.

Meanwhile, Pakistani security forces were on high alert for sectarian violence ahead of the start, expected Friday, of the holy month of Moharram.

"There is a threat perception and we are not going to take any chances. Our security apparatus will be on the highest alert," interior ministry spokesman Javed Cheema told AFP.

Bhutto's murder, the delay of general elections until February 18 and a sharp rise in Islamist militancy have intensified the sense of instability at a time of traditional tension between minority Shiites and majority Sunnis.

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