WASHINGTON - The killing of former Afghan president Burhanuddin Rabbani and other assassinations by insurgents are strategically "significant" but will not force a change in war strategy, the US military's top officer said Tuesday. US Defense Secretary Leon Panetta and Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, described the death of Rabbani -- who was leading peace efforts in Afghanistan -- as part of a campaign of high-profile assassinations by the Taliban triggered by a lack of success on the battlefield.

Mullen said insurgents have "not succeeded on the ground this year. Their campaign has failed in that regard.

"They've shifted to these high-profile attacks," he told a news conference.

The admiral said the US military took the killings seriously.

"Strategically, they're significant," he said.

But Mullen added that there were no plans to shift the overall war strategy in the nearly ten-year-old war as a result of the slayings.

Panetta, speaking at the same press conference, said the assassination tactic was a "concern" and that US commanders were working with Afghan forces to try to thwart the Taliban.

But he insisted the US-led war effort was making headway.

"And we've got to take steps to try to make sure that we protect against that, and we're in the process of doing that," Panetta said.

"But the bottom line still remains that we are moving in the right direction. 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," he said.

Rabbani's death is the most high-profile political assassination since the 2001 US-led invasion that ousted the Taliban from power.

The former president, head of Afghanistan's High Peace Council established last year by President Hamid Karzai, was killed Tuesday by a Taliban suicide bomber with explosives hidden in his turban. (AFP)