Prime Minister Tony Blair (search) said Tuesday it was still unclear what caused a British military transport plane to crash north of Baghdad, killing all 10 servicemen on board in Britain's biggest single loss of life in the Iraq conflict.

Insurgents claim to have shot down the C-130 Hercules (search) aircraft north of Baghdad on Sunday, and Al-Jazeera television has aired a videotape from guerrillas showing flaming wreckage of a plane.

"Let me again express my sympathy and condolences to the families of those that have died," Blair told the ITV television station.

"We don't yet know the exact cause of the crash. We hope we will be able to give people more details of that in due course."

The nine Royal Air Force (search) personnel and one soldier on board the aircraft died, a spokesman for the Ministry of Defense said Tuesday. They had previously been reported missing and presumed dead.

The deaths brought the British military's death toll from the Iraq war to 86. The highest previous death toll from a single incident was eight British servicemembers, killed in a helicopter crash in which four Americans also died in March 2003.

Britain's Chief of the Air Staff, Air Chief Marshal Sir Jock Stirrup, said a senior investigator was on his way to Iraq, where staff on the ground were already sifting through the wreckage of the C-130.

The Hercules was on an administrative flight between Baghdad and a U.S. air base at Balad when it went down 19 miles northwest of Baghdad, the Ministry of Defense has said. The crash came on election day, which insurgents had pledged to disrupt.

The authenticity of the video shown on Al-Jazeera could not be confirmed. It showed a finger pressing a button on a white box with wires, and then images of two missiles or rockets flying up into the air. The video did not show any impact with a plane. Instead, it cut to footage of people in street clothes walking through a plane's wreckage burning on the ground.

It was not immediately possible to determine if the wreckage shown was that of the crashed Hercules. In London, officials at the Foreign Office said they were aware of the reported video, but had no further comment.

A spokesman for Al-Jazeera said the television station received the video from an Islamic militant group that called itself "the Green Brigade." An expert said Tuesday that the images weren't representative of anything that could shoot down a Hercules.

"The missile shown looks more like a multiple-launched rocket system, which isn't designed to shoot down aircraft," said Jim O'Halloran, editor of Jane's Land-Based Air Defense.

A different group, Ansar al-Islam (search), claimed in a Web statement that its fighters had shot down the plane with an anti-tank missile. That group provided no evidence of the claim.

O'Halloran said an anti-tank missile wouldn't have been able to shoot down a Hercules if it was flying at an altitude of 15,000 feet, as reported in the British media.

A Ministry of Defense spokeswoman was unable to confirm that the aircraft was flying at that altitude, or reports that the plane came down in clear skies.