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 on Wednesday , 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 can't accept that and I will do everything I possibly can to prevent that specifically," he said.

"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.