The United States has led top secret efforts to remove highly enriched uranium from Pakistan for years, worried it could be used to make an "illicit" nuclear device, according to leaked US cables.
The New York Times said they were among quarter of a million confidential American diplomatic cables released by whistleblower WikiLeaks in what Pakistan condemned as an "irresponsible disclosure of sensitive official doucments".
The country's nuclear arsenal is one of the most sensitive topics for the United States as it tries to improve relations with the conservative muslim nation on the front line in the campaign against Al-Qaeda and the Taliban.
Parts of Pakistan's northwest is gripped by a homegrown Taliban insurgency. Its semi-autonomous wild border area with Afghanistan is subject to a covert US drone war targeting Taliban and Al-Qaeda leaders.
Islamist militants embarked on a nationwide bombing campaign across Pakistan in 2007, the same year that the times said the secret efforts began.
In May 2009, it quoted then US ambassador Anne Patterson as saying that Pakistan was refusing to schedule a visit by American technical experts.
The Times attributed the reason to a nameless Pakistani official who said: "If the local media got word of the fuel removal, 'they certainly would portray it as the United States taking Pakistan's nuclear weapons'".
Islamabad has been adamant that its nuclear weapons are in safe hands and US President Barack Obama has publicly concurred.
But the Times said the leaked documents showed the United States trying to remove the uranium from a research reactor, fearing it could be diverted for use in an "illicit nuclear device".
The newspaper did not elaborate on how the United States had sought to remove the uranium or the nature of any such device.
Experts estimate that Pakistan already has up to 100 nuclear weapons.
Pakistan's foreign ministry said it had been offically informed of the leaks by the US government in advance.
"At this stage we are examining the relevant documents and their contents," ministry spokesman Abdul Basit told AFP when asked about the Times report.
The United States has longstanding concerns about proliferation from Pakistan and is reported to have set up an elite squad that could fly into the country and attempt to secure its weapons should the government disintegrate.
Pakistan announced that it had nuclear weapons in 1998, scrambling to secure the technology after India's first nuclear test in 1974.
Western analysts believe China assisted Pakistan in developing the Khushab nuclear site to produce plutonium, which can be miniaturised for cruise missiles — presumably aimed at India.
In 2004, Abdul Qadeer Khan, the father of Pakistan's bomb, confessed to running a nuclear black market that sent secrets to Iran, Libya and North Korea. He was put under house arrest for five years.
Although he retracted his remarks, US officials say he is still a risk.
Pakistan also opposes a proposed fissile material cutoff treaty, which would limit access to highly enriched uranium and plutonium used to make weapons.
Pakistan says the treaty would give a permanent nuclear imbalance to India, with which it has fought three full-fledged wars since independence in 1947.