Notwithstanding Pakistan's repeated assertions that its nuclear arsenal was out of the reach of militants, the US has sought more assurances warning that it would be "the very very worst case scenario" if the Islamic state's atomic weapons fall into the Taliban's hands.
Ahead of President Asif Ali Zardari's talks here with his US counterpart Barack Obama as part of a trilateral meet also involving Afghanistan, National Security Adviser Gen James Jones said Pakistan army had repeatedly told him the stockpile was "under control", but "this is very much an ongoing topic".
"Things are moving in a more positive direction" in Pakistan, but more assurances were needed about the safety of the country's nuclear weapons, Jones told BBC in an interview.
"If Pakistan doesn't continue in the direction that it presently is and we're not successful there then, obviously, the nuclear question comes into view."
The US National Security Adviser's comments came as Pakistan government scrambled hard to prevent an emboldened Taliban from expanding their influence in the country's restive northwest bordering Afghanistan.
Pakistan's nuclear weapons falling into the hands of the Taliban would be "the very very worst case scenario," said Gen Jones. "We're going to do anything we can within the construct of our bilateral relations and multilateral relations to make sure that doesn't happen," he said.
"We have received many assurances from the military that this is something they have under control but this is very much an ongoing topic," Jones said.
"The world would like to know that on this question, that there's absolute security and transparency." However, his comments were in contrast to State Department spokesman Robert Wood's statement that Pakistan's nuclear arsenal was dispersed and that there was a very solid command and control structure in place.
"Pakistan's nuclear arsenal is dispersed. There, as I said, is a very solid command and control structure in place with regard to that arsenal," he said.
Zardari, Obama and Afghan President Hamid Karzai are due to meet at the White House on Wednesday to discuss a strategy to deal with terrorism in Pakistan and Afghanistan.
Turning to Iran, Jones warned that the US would not "wait forever" for Tehran to respond positively to President Obama's attempt to improve relations with the country.
"We're interested in getting this new relationship established, to the extent that there is going to be one, but it takes two - and we're still waiting for the appropriate response from the Iranians," he told BBC.
