Washington: Piling up pressure on Pakistan, the US has asked its authorities, particularly ISI, to sever ties with the Haqqani network of Taliban and demanded "strong and immediate action" against the outfit, even as it insisted there is no permanent breach in bilateral relations and lines of communication are open.

"It is critical that the government of Pakistan break any links they have, and take strong and immediate action against this network so that they are no longer a threat to the United States or to the people of Pakistan -- because this network is a threat to both," White House Press Secretary Jay Carney told reporters on Friday evening.

Carney said the Obama administration believes and knows that the Haqqani network was responsible for the attacks on the US embassy in Kabul and the International Security Assistance Force (ISAF) headquarters, and a number of other recent attacks that caused death and injury not only to Afghans but to American soldiers.

"We know that the Haqqani network operates from safe havens in Pakistan and that the government of Pakistan has not taken action against these safe havens," he said, adding that top US officials, including Secretary of State Hillary Clinton and Chairman of the Joint Chiefs of Staff Admiral Mike Mullen, would continue to have conversations with the Pakistani leadership on this issue.

The United States, Carney said, 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.

Amidst a war of words between the US and Pakistan on Haqqani network, 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.

The United States and Pakistan, both of them, have been the victims of terrorism and need to continue to work together on that problem, he 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.

Captain John Kirby, spokesperson of Admiral Mullen, told Pentagon reporters that there has been a long-standing historical relationship between the ISI and extremist groups.

"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."

However, Kirby said Mullen has not spoken to Kayani after his Congressional testimony on Thursday in which he described the Haqqani network as a "veritable arm" of the ISI.

“We still want to pursue a working, productive partnership with the Pakistani military, and that in large measure depends on their willingness and their ability to disconnect themselves from extremist groups like the Haqqani network," Kirby said.

But, he conceded that this is a "tough relationship that requires a lot of energy" and lot of dialogue.