WASHINGTON (Reuters) - U.S. President Barack Obama said on Friday Pakistan's relationship to the militant Haqqani network is unclear, but he urged Islamabad to curb any active or passive support for that Taliban faction.

"The intelligence is not as clear as we might like in terms of what exactly that relationship is," Obama said in a radio interview, when asked about an issue at the heart of a bitter row between Washington and Islamabad.

"But my attitude is, whether there is active engagement with Haqqani on the part of the Pakistanis or rather just passively allowing them to operate with impunity in some of these border regions, they've got to take care of this problem," he told radio host Michael Smerconish.

The United States and Pakistan have been bickering publicly during the last week after outgoing chairman of the Joint Chiefs of Staff Admiral Mike Mullen said a violent Taliban faction was a "veritable arm" of the Pakistani spy service.

Asked if Mullen was correct, Obama said: "I think Mike's testimony expressed frustration over the fact that safe havens exist, including the al Haqqani network safe haven, inside of Pakistan."

Obama said Washington would remain firm with Pakistan on the safe haven issue, "but we've tried to also preserve the intelligence cooperation that we've obtained that's allowed us to go after al Qaeda in a very effective way."

The U.S. president credited Pakistan with "outstanding cooperation in going after al Qaeda" and vowed to keep working with Islamabad on the militant issue.

"There's no doubt that the relationship is not where it needs to be and we are going to keep on pressing them to recognize that it is in their interest -- not just ours -- to make sure that extremists are not operating within their borders," Obama added.

(Reporting by Paul Eckert, Editing by Sandra Maler and Vicki Allen)