While he acknowledged that there is a limit to what the United States knows about the nuclear weapons, he believes that Pakistan's military leaders understand the threat if it falls into the insurgents hands.
"I know what we've done over the last three years, specifically, to both invest, assist (Pakistan), and I've watched them improve their security fairly dramatically over the last three years," Mullen said.
He said he does not believe that Pakistan's nuclear weapons will fall into the hands of terrorists. But when asked if he would say he is confident -- rather than just comfortable -- with the state of Pakistan's nuclear security, he stuck with the latter.
Instead, Mullen said his greater worry is Pakistan's ability to sustain their military operations, as Taliban violence surges in the region.
"I'm gravely concerned about the progress they (the Taliban) have made in the south and inside Pakistan," Mullen said. "The consequences of their success directly threaten our national interests in the region and our safety here at home."
U.S. administration and military leaders have said that success in the Afghanistan war is linked to security in Pakistan. And officials will meet this week with leaders from both countries. Part of those meetings will focus on setting benchmarks for economic, political and military progress there.
Mullen would not detail the benchmarks under discussion, but he said that the U.S. must have patience as it works to solidify a relationship with Pakistan, that can lead to a more secure region.
"We're just going through a very hard time right now in building it, and that's going to take considerable effort," said Mullen, adding that it also will take "an extended period of time to get this right."
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