Washington: Jarred by the brazen assassination of top Afghan leader Burhanuddin Rabbani in heavily secured area of Afghan capital Kabul, US has asked Pakistan's powerful spy agency ISI to sever ties with terror outfits particularly the Haqqani network.
     
Washington has also vowed to put "as much pressure as possible" on Islamabad to control these insurgent groups on their side of the border.
     
"The ISI has been supporting proxies for an extended period of time... I think that strategic approach has to shift in the future", said Mike Mullen, Chairman of the US Joint Chiefs of Staff.
     
While world leaders said Rabbani's death would not derail Afghan peace process, the US military said the attack signalled that the Taliban was changing tactics by moving off the battlefield to go after high-profile targets elsewhere.
      
"Strategically, they're significant," said Mullen, referring to a series of Taliban suicide bombings over the summer that killed, among others, President Karzai's half brother and recent bold attacks near the US embassy and NATO headquarters in the Afghan capital.
     
Mullen and Defense Secretary Leon Panetta who were speaking at a Pentagon press briefing here said that the assassination tactic was a "concern" and US commanders were working with Afghan forces to try to thwart the Taliban.
     
"The bottom line still remains that we are moving in the right direction," Panetta added.
     
Mullen, the outgoing US military Chief said he had raised the issue of Haqqani network strongly with Pakistan's Army Chief Gen Ashfaq Parvez Kayani in Spain.

"I addressed this issue very strongly with General Kayani the other night, when I met him. It was the heart of the discussion, that the Haqqani --
the proxy connection to the ISI, the Haqqanis working across border, killing our people, killing Afghans-- has to stop," he said.
     
"That's not a new message, but it's one that he clearly understands, and I think it's one we have to keep reiterating," Mullen said in response to a question.
     
Defense Secretary Panetta said, Islamabad must understand that terrorism was as much as threat for Pakistan as it is for the US.
     
"We keep telling them, you can't choose among terrorists. If you're against terrorism, you have to be against all forms of terrorism. And that's something we just have to continue to stress," he added.
   
Both the top US officials said they had absolutely no doubt that the Haqqani network was behind recent brazen attacks in Kabul.
     
Though no group has claimed responsibility for the killing of the former Afghan President, several groups could have been involved, including the the Haqqani Network, a terrorist organisation based in Pakistan’s tribal areas and with affiliations to the country’s intelligence agency, ISI; and even elements of Al Qaeda, given the method and precise and long-term planning involved in the assassination.
     
The paper said the attack plot was similar to assassination of the most celebrated Afghan mujahaeddin commander Ahmad Shah Masood.
     
The Defence Secretary's comments came shortly after a suicide bomber laden with explosives in his turban on Tuesday assassinated Afghanistan's former President who was leading efforts to talk peace with insurgents.
   
Panetta said the US has made progress in weakening the Taliban and al-Qaeda.
   
"We have made progress in going after their leadership. But they now are resorting to these kinds of attacks, to these kinds of high-level assassinations, which, are of concern. We have to take steps to try to make sure that we protect against that.”
    
"We are in the process of doing that. We're working with the Afghans to try to discuss with them steps on how we can provide better protection so that this does not occur. But the bottom line still remains that we are moving in the right direction. And we can't let some of these sporadic events deter us from the progress that we've made," he said.

(Agencies)