KABUL (AFP) – A British aid worker killed during a failed US rescue operation in Afghanistan had faced an imminent threat of being killed by her kidnappers or smuggled into Pakistan, Western officials said on Sunday.

Linda Norgrove, 36, was working for US development group DAI when she and three Afghan staff were captured on September 26 while travelling in Kunar, a hotbed of Taliban activity in eastern Afghanistan bordering Pakistan.

US forces launched an operation to rescue her late Friday, but she died during the attempt.

"All the information we have is that she was killed by an explosion, most likely a suicide vest held by a hostage taker," a British government source told AFP on condition of anonymity.

"Nothing at all suggests US fire was the cause of the death."

On Saturday an Afghan intelligence official had suggested that Norgrove was killed by a grenade thrown into the room where she was being held as the US forces approached. The troops then killed all her captors, the official said.

The British government and NATO both said that it had been a difficult decision to take, but stressed that her life had been in grave danger.

"Where a British life is in such danger, and where we and our allies can act, I believe it is right to try. I pay tribute to the courage and skill of all those involved in this effort," said Prime Minister David Cameron.

A Western official speaking on condition of anonymity in Kabul said the mission was ordered because Norgrove was under imminent threat -- "either being killed or being taken to Pakistan".

Once hostages are smuggled across the border into Pakistan, particularly the tribal belt that lies outside direct government control, tracking their whereabouts becomes far more difficult.

Pakistani officials have described the tribal badlands, a suspected hiding place of Al-Qaeda mastermind Osama bin Laden, as an intelligence black hole where sharp ravines, mountains and thick forest provide perfect cover.

Western intelligence sources have said that the prospect of transferring a hostage to Pakistan is a cause for attempting a rescue.

"I don't know if it was a bomb, a grenade, belt or anything. But the captors killed her. US special forces who were involved didn't kill her. She was inside the room. They really did their best," said the Western official.

US General David Petraeus, the commander of American and NATO troops in Afghanistan, said troops did "everything in their power to rescue Linda."

Norgrove's father, John, told the Sunday Times he was "devastated" by his daughter's death.

Speaking from the family's home in the Outer Hebrides in Scotland, he said: "We've only known for a few hours -- we are devastated."

The United Nations, for whom Norgrove worked previously for three years in Afghanistan, condemned her killing and the deteriorating security situation in which aid workers are operating.

"The increasing violence which targets humanitarian and development workers, as well as civilians in Afghanistan is unacceptable," said humanitarian and UN residential coordinator Robert Watkins.

Norgrove's aid worker colleagues also paid tribute to her work.

"Linda loved Afghanistan and cared deeply for its people, and she was deeply committed to her development mission. She was an inspiration to many of us here at DAI and she will be deeply missed," said DAI president James Boomgard.

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