President Hamid Karzai has sought tribal support for a NATO-led offensive against the Taleban in their southern heartland to try and turn the tide in a conflict that has dragged on for more than nine years.

Linda Norgrove, 36, who worked for a US aid group, had been abducted Sept. 26 along with three Afghan coworkers when they visited a project in a remote part of Kunar province, a lawless region bordering Pakistan.

"Responsibility for this tragic outcome rests squarely with the hostage takers. From the moment they took her, her life was under grave threat," British Foreign Secretary William Hague said in a statement Saturday.

He gave no further details on the Friday night rescue bid. But a British Foreign Office source said US troops had attempted the rescue. Norgrove, an ex-UN worker, headed a $150 million US aid project designed to build local economies.

Her death highlights the increasing dangers faced by aid workers in Afghanistan, where insurgents and other armed groups hold sway in many parts of the country. "This is devastating news," said James Boomgard, president of her organization, DAI, a private company involved in development.

In August, eight foreign medical workers, including a British doctor, as well as two Afghans, were killed by unidentified gunmen in the remote northeast. Insurgents are still holding two French journalists seized last December.

The rescue attempt was not the first such operation. A raid that freed New York Times reporter Stephen Farrell, a Briton, from his Afghan captors last year provoked anger after his Afghan colleague and a British soldier were killed.

The Afghan war is weighing increasingly on US President Barack Obama's administration as he and his NATO allies face pressure at home to end the unpopular war.