Arlington, Virginia:
The top US military officer said on Monday that he was comfortable that Pakistan's nuclear weapons remained secure, but is gravely concerned about Taliban advances there and in Afghanistan.
Admiral Mike Mullen, chairman of the US Joint Chiefs of Staff, told reporters that the United States had worked with the Pakistanis to improve the security of their nuclear arsenal, and believed that country's military was focused on keeping them secure.
While he acknowledged that there was a limit to what the United States knew about the nuclear weapons, he believed that Pakistan's military leaders would not let them fall into insurgents' hands.
Instead, Mullen said his greater worry was Pakistan's ability to sustain its military operations in the face of a surge in Taliban violence in the region.
"I'm gravely concerned about the progress they 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."
The US administration and military leaders have said that success in the Afghanistan war is linked to security in Pakistan, and US officials will meet this week with leaders from both countries.
The meetings will include the issue of setting benchmarks for economic, political and military progress there.
Mullen would not detail the benchmarks under discussion, but he said that the US must have patience as it works to solidify a relationship with Pakistan, that can lead to a more secure region.
Pakistan's President Asif Ali Zardari will meet President Barack Obama in Washington on Wednesday.
Available information suggests that Pakistan's secret nuclear sites are protected by crack troops and multiple physical barriers, making the risk from an outright Taliban attack relatively low.
A more worrying prospect for some experts is possible infiltration by radical Islamists of Pakistan's nuclear facilities.
Stringent security checks on personnel are meant to prevent that as well. But Pakistan's nuclear establishment has seen serious leaks of nuclear knowledge and materials by insiders in the past.
Top government scientist AQ Khan operated a global black market nuclear network for more than a decade until he was uncloaked by U.S. intelligence. And the CIA has confirmed a meeting between Khan associates and Osama bin Laden before the September 11, 2001 terror attacks.
The issue of Pakistan's nuclear security is expected to come up during general discussions between Zardari and Obama on the Taliban insurgency.