The country insists smaller weapons would deter a sudden attack by its bigger neighbour India. But the United States worries tactical weapons may further destabilise an already volatile region.
Sharif and Obama are due to meet on Thursday.
The United States wants Pakistan to commit to not using tactical nuclear weapons but Islamabad wants to keep its options open as a way of deterring a potential Indian attack, said Maria Sultan, head of the South Asian Strategic Stability Institute.
PM Nawaz arrives in Washington on official visit
Pakistan says the United States is demanding unreasonable limits on its use of nuclear weapons and not offering much in return apart from a hazy promise to consider Pakistan as a recognised recipient of nuclear technology.
"Pakistan’s nuclear programme is ... India-centric. And it exists to make war a non-option ... Tactical nuclear weapons block off this room (for war) completely," said a security official with knowledge of Pakistan's nuclear programme. "No one can dictate what kind of weapons we will make or use."
Pakistan was working on developing a nuclear submarine, he added. "The goal is a sea-based second strike capability," he said, referring to a submarine that could carry nuclear warheads and strike in case land-based nuclear weapons were wiped out.
Foreign Secretary Aizaz Chaudhry told state television on Tuesday this was a reaction to Indian threats to make a limited, lightning raid with conventional forces in case of militant attack, an idea known as the "Cold Start" doctrine.
"In India, they brought the cold start doctrine," he said. "So we have also preserved our deterrence capability."
Non-proliferation experts worry that country’s tactical nuclear weapons further destabilise an already volatile region.
"The smaller they (nuclear weapons) are, the more tempting it becomes to use them against a conventional force," said nuclear physics professor Pervez Hoodbhoy.
"The development and deployment of tactical nuclear weapons is a complete change of strategy. Earlier, nuclear weapons were instruments for deterring war, but now they’re seen as weapons for actually fighting a war."
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