The United States is also frustrated the army of the unstable but nuclear-armed Pakistan has not been able to stop extremists in these areas from launching cross-border attacks into Afghanistan against U.S. and NATO troops, the Financial Times reported Friday.

It is these concerns that brought Mullen to Pakistan for the fifth time in one year since becoming chairman of the U.S. Joint Chiefs of Staff, the report said.

In each of these trips, the Admiral sought to convince Pakistan Military Chief Gen. Ashfaq Kayani for stronger measures against the militants, who also have not spared Pakistani institutions, the report said.

"What is going on, or not going on actually, in the tribal areas of Pakistan is a major contributing factor to what you see in Afghanistan," a US official told the newspaper. "The past year has seen this full maturing of the safe haven in Pakistan."

The developments have caused violence in Afghanistan to escalate to post-invasion high and drawn U.S. attention closer to that country.

Speaking to U.S. Congress last week, Mullen said the country is not winning the war. The U.S. is aware too much pressure on the Pakistani military could strengthen anti-U.S. feelings in that country, the report said.

The recent incursion by U.S. Special Forces inside Pakistan also has strained relations.