( CNN ) -- Responding to an allegation by the U.S. chairman of the Joint Chiefs of Staff that Pakistan 's main intelligence agency has a `` longstanding relationship '' with a Taliban-allied insurgent group that targets U.S. troops in Afghanistan , a senior Pakistani intelligence official said : `` We do have a relationship : that of an adversary . '' `` We have made our resolve very clear that ( the Haqqani Network ) is an enemy we need to fight together , '' said the official , who did not want to be identified discussing intelligence matters . In an interview that aired Wednesday on Pakistan 's Geo TV , Adm. Michael Mullen spoke forcefully about the Haqqani Network , which he said `` very specifically facilitates and supports the Taliban who move in Afghanistan , and they 're killing Americans . '' `` I ca n't accept that and I will do everything I possibly can to prevent that specifically , '' he said . Then Mullen said Pakistan 's Inter-Services Intelligence `` has a longstanding relationship with the Haqqani Network . That does n't mean everybody in the ISI , but it 's there . '' `` I also have an understanding that the ISI and the ( Pakistani military ) exist to protect their own citizens , and there 's a way they have done that for a long period of time , '' Mullen said . `` I believe that over time , that 's got to change . '' Based in Pakistan 's North Waziristan frontier , the Haqqani Network `` has been at the forefront of insurgent activity in Afghanistan , responsible for many high-profile attacks , '' according to the United Nations . The group is believed to have three main sources of funds : donations from the Persian Gulf region , drug trafficking , and al Qaeda payments . Pakistani forces in December announced they had seized Nasiruddin Haqqani , son of the group 's leader , Jalaluddin Haqqani . Other U.S. officials in recent months had expressed concerns that Pakistan has not been aggressively confronting militants operating in the tribal regions . The Pakistani intelligence official told CNN that `` we have our hands full '' fighting other Islamist militant groups along the border with Afghanistan , notably those under the umbrella of the Lashkar-e-Tayyiba ( LeT ) `` and once we are through with them we can turn on the other ( the Haqqanis ) . We do not have the capacity to undertake simultaneous operations . '' The official said the `` onus of providing proof of this '' relationship was on the Americans and it was not up to the ISI `` to start providing clarification . '' Asked if offense was taken by Mullen 's remarks , the intelligence official said : `` Not personally , no . '' Earlier in the Geo TV interview , Mullen was pressed on issues such as the arrest of CIA contractor Raymond Davis and unmanned drone strikes against targets within Pakistan , which have marked what he called a `` rough patch '' of increasingly complex relations between the two nations in recent months . The admiral traced the difficulties back to the 12-year period in which the two countries had severed diplomatic ties , then restored them in the midst of the global terrorism crisis that followed the attacks of September 11 , 2001 . `` We ca n't snap our fingers and say all of the sudden we trust each other , '' he said , `` and that 's what we 're trying to work our way through in the midst of these huge terrorist challenges that we both have . '' `` It 's the focus from the United States ' perspective on the terrorist threat ( in the Federally Administered Tribal Areas ) , the al Qaeda leadership which still lives there and still threatens to kill as many Americans as they possibly could , combined with what I would call this federation of terrorist organizations that are getting along more than they used to , and at least from my experience ... the complexity is increasing , not decreasing . '' As Joint Chiefs chairman , Mullen is the highest-ranking uniformed official in the U.S. military 's chain of command . He has been part of recent discussions that have included CIA Director Leon Panetta and their Pakistani counterparts : Chief of Army Staff Gen. Ashfaq Parvez Kayani and the ISI 's director , Lt. Gen. Ahmad Shuja Pasha . Mullen 's term is set to end this year . `` It 's been a very rough patch lately , '' Mullen said , `` and I think the leaders , including Generals Pasha and Kayani , Director Panetta and myself and others are very committed to working our way through this because we see the need to solve this problem , and we just ca n't walk away from it . '' CNN 's Nick Paton Walsh , Zarifmo Aslamshoyeva and Nasir Habib contributed to this report .
Adm. Mike Mullen said ISI has `` longstanding relationship '' with the Haqqani Network . The U.S. military is also investigating whether Pakistani officials were involved in a plot to assassinate former Afghan President Hamid Karzai and his family last year. Despite strain , Mullen says leaders `` are very committed to working our way through this ''