Demanding a "strong and immediate action" against the outfit, US officials said Haqqani group's activities have become "more brazen, more aggressive and more lethal".
Captain John Kirby, spokesperson of Chairman Joint Chiefs of Staff Admiral Mullen, told Pentagon reporters on Friday evening that there has been a long-standing historical relationship between the ISI and extremist groups.
Kirby said: "There has been a lot of activity over the course of this summer the hotel attack in Kabul, Wardak truck bomb, attack on the embassy and others smaller level operations. It has been a very busy summer for the Haqqani network. It has gone worse.
"Their (Haqqani's) activities have become more brazen, more aggressive and more lethal. Information has become more available that these attacks have been supported or encouraged by the ISI. The Chairman (Mullen) just had the conversation with (Pakistan army chief) General (Ashfaq Parvez) Kayani last week in Spain. He said this because this is the truth."
"All I can tell you that we are confident that the ISI continues to support and even encourages the Haqqanis to launch these attacks. I am not going into, at this point, specifics of the intelligence that we have about the support they have we believe," he said.
However, Kirby said Mullen has not spoken to Pakistan army chief Gen Ashfaq Pavrvez Kayani after his Congressional testimony on Thursday in which he described the Haqqani network as a "veritable arm" of the ISI.
Meanwhile, White House Press Secretary Jay Carney said it
was critical that Pakistan break any links they have with Haqqanis, and take strong and immediate action against this network so that they are no longer a threat.
But, Carney said, US has an important relationship with Pakistan. "That relationship and the cooperation that we have had with Pakistan has assisted us greatly in our efforts to defeat al-Qaeda. It is important to remember that Pakistan has suffered mightily at the hands of terrorists and they've paid a terrible price for it," he said.
"It is a complicated relationship, as you've heard me say before, and when we have issues that we need to discuss with the Pakistanis we're very candid and forthright in doing so," Carney said.
A Pentagon official stressed that the lines of communication with Islamabad are open and there is no permanent breach in relationship with Islamabad.
"The lines of communication with our Pakistani counterparts remain open. This is a relationship that's complicated but essential," Pentagon spokesman George Little told reporters. "We have disagreements, sometimes serious ones. That does not mean that we are going to stop dialogue. It is important that we find cooperation on issues that are of interest to us and fighting terrorism is one of them," Little said.
"I wouldn't take you to the point where you would suggest some kind of permanent breach. That is not the case. The lines of communication remain open," said the Pentagon spokesman in response to a question.