The head of the U.S. Joint Chiefs of Staff said he believes that U.S. and Afghan troops must adapt their tactics to prepare for more audacious attacks by the Taliban.
U.S. Admiral Mike Mullen made the comments at a Washington press conference hours after the head of the Afghan High Peace Council and former Afghan President Burhanuddin Rabbani was assassinated in a suicide attack in his Kabul home.
It was the fourth significant insurgent attack since June in the Afghan capital. Blame for the first three has been ascribed to the Haqqani Network, a Taliban-allied insurgent group operating from a base on the Afghan-Pakistan border.
The Taliban has refuted reports that it was involved in Rabbani's death.
At the same news conference, U.S. Defense Secretary Leon Panetta said he agreed with Mullen that the Taliban has shifted to a tactic of assassinations and sieges recently because they are not succeeding on the battlefield.
But he insisted the U.S.-led war effort was making headway.
"The bottom line still remains that we are moving in the right direction," he said. "We have made progress against the Taliban and we can't let some of these sporadic events deter us from the progress that we've made."
Earlier at a think tank event, Mullen told RFE/RL that it's too early to predict what impact Rabbani's death will have on the year-old effort to find a political solution to the conflict:
"Obviously, as the chairman of the High Peace Council, in that regard certainly someone is going to have to step in pretty quickly, because that's a critical part of the entire peace process."
Afghan President Hamid Karzai said the assassination "will not deter" him from seeking a negotiated settlement to the war.