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 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.