The Pakistanis have significantly accelerated production of uranium and plutonium for bombs and developed new weapons to deliver them. After years of approximate weapons parity, experts said, Pakistan has now edged ahead of India, its nuclear-armed rival.
An escalation of the South Asian arms race poses a dilemma for the Obama administration, which has worked to improve its economic, political and defense ties with India, while seeking to deepen its relationship with Pakistan as a crucial component of its Afghanistan war strategy.
In politically fragile Pakistan, the administration is caught between fears of proliferation or possible terrorist attempts to seize nuclear materials and Pakistani suspicions that the United States aims to control or limit its weapons program and favors India.
Those suspicions were on public display last week at the opening session of U.N. disarmament talks in Geneva, where Pakistani Ambassador Zamir Akram accused the United States and other major powers of "double standards and discrimination" for pushing a global treaty banning all future production of weapons-grade uranium and plutonium.
Adoption of what is known as the "fissile materials cutoff treaty," a key element of President Obama's worldwide non-proliferation agenda, requires international consensus. Pakistan has long been the lone holdout.
While Pakistan has produced more nuclear-armed weapons, India is believed to have larger existing stockpiles of such fissile material for future weapons. That long-term Indian advantage, Pakistan has charged, was further enhanced by a 2008 U.S.-India civil nuclear cooperation agreement. The administration has deflected Pakistan's demands for a similar deal.
Brig. Gen. Nazir Butt, defense attache at the Pakistani Embassy in Washington, said the number of Pakistan's weapons and the status of its production facilities were confidential.
"Pakistan lives in a tough neighborhood and will never be oblivious to its security needs," Butt said. "As a nuclear power, we are very confident of our deterrent capabilities."
But the administration's determination to bring the fissile materials ban to completion this year may compel it to confront more directly the issue of proliferation in South Asia. As U.S. arms negotiator Rose Gottemoeller told Bloomberg news at the U.N. conference Thursday: "Patience is running out."
Other nuclear powers have their own interests in the region. China, which sees India as a major regional competitor, has major investments in Pakistan and a commitment to supply it with at least two nuclear-energy reactors.
Russia has increased its cooperation with India and told Pakistan last week that it was "disturbed" about its arms buildup.
"It's a risky path, particularly for a government under pressure," Deputy Foreign Minister Sergey Ryabkov, fresh from a visit to Islamabad, said during remarks at the Nixon Center on Thursday.
Wary of upsetting Pakistan's always-fragile political balance, the White House rarely mentions the country's arsenal in public except to voice confidence in its strong internal safeguards, with warheads kept separate from delivery vehicles. But the level of U.S. concern was reflected during last month's White House war review, when Pakistan's nuclear security was set as one of two long-term strategy objectives there, along with the defeat of al-Qaeda, according to a senior administration official who spoke on condition of anonymity.
A publicly released summary of the classified review document made no reference to the nuclear issue, and the White House deflected questions on grounds that it was an intelligence matter. This week, a spokesman said the administration would not respond to inquiries about the size of Pakistan's nuclear arsenal.
National Security Council spokesman Tommy Vietor referred to Obama's assurance at last spring's Nuclear Security Summit that he felt "confident about Pakistan's security around its nuclear weapons program." Vietor noted that Obama hs encouraged "all nations" to support negotiations on the fissile cutoff treaty.
"The administration is always trying to keep people from talking about this knowledgeably," said David Albright, president of the Institute for Science and International Security and a leading analyst on the world's nuclear forces. "They're always trying to downplay" the numbers and insisting that "it's smaller than you think."
"It's hard to say how much the U.S. knows," said Hans M. Kristensen, director of the nuclear information project at the Federation of American Scientists and author of the annual global nuclear weapons inventory published in the Bulletin of Atomic Scientists. "Probably a fair amount. But it's a mixed bag - Pakistan is an ally, and they can't undercut it with a statement of concern in public."
Beyond intelligence on the ground, U.S. officials assess Pakistan's nuclear weapons program with the same tools used by the outside experts - satellite photos of nuclear-related installations, estimates of fissile-material production and weapons development, and publicly available statements and facts.
Four years ago, the Pakistani arsenal was estimated at 30 to 60 weapons.
"They have been expanding pretty rapidly," Albright said. Based on recently accelerated production of plutonium and highly enriched uranium, "they could have more than doubled in that period," with current estimates of up to 110 weapons.
Kristensen said it was "not unreasonable" to say that Pakistan has now produced at least 100 weapons. Shaun Gregory, director of the Pakistan Security Research Unit at Britain's University of Bradford, put the number at between 100 and 110.
Some Pakistani officials have intimated they have even more. But just as the United States has a vested interest in publicly downplaying the total, Pakistan sees advantage in "playing up the number of weapons they've got," Gregory said. "They're at a disadvantage with India with conventional forces," in terms of both weaponry and personnel.
Only three nuclear countries - Pakistan, India and Israel - have never signed the nuclear non-proliferation treaty. India is estimated to have 60 to 100 weapons; numbers are even less precise for Israel's undeclared program, estimated at up to 200. North Korea, which has conducted nuclear tests and is believed to have produced enough fissile material for at least a half-dozen bombs, withdrew from the treaty in 2003.
Those figures make Pakistan the world's fifth largest nuclear power, ahead of "legal" powers France and Britain. The vast bulk of nuclear stockpiles are held by the United States and Russia, followed by China.
While Pakistan has no declared nuclear doctrine, it sees its arsenal as a deterrent to an attack by the Indian forces that are heavily deployed near its border. India has vowed no first-use of nuclear weapons, but it depends on its second-strike capability to deter the Pakistanis.
The United States imposed nuclear-related sanctions on both Pakistan and India after weapons tests in 1998, but lifted them shortly after the terrorist attacks of Sept. 11, 2001. With U.S. guidance and a $100 million assistance program, Pakistan moved to increase international confidence by overhauling its command and control structures.
Revelations in 2004 about an illegal international nuclear procurement network run by Pakistani nuclear official A.Q. Khan, which supplied nuclear materials to Libya, Iran and North Korea, led to further steps to improve security.
The 2008 agreement that permits India to purchase nuclear fuel for civilian purposes was a spur to Pakistani weapons production, experts said. Pakistan maintains that the treaty allows India to divert more of its own resources for military use.
As Pakistan sees India becoming a great power, "nuclear weapons become a very attractive psychological equalizer," said George Perkovich, vice president for studies and a non-proliferation specialist at the Carnegie Endowment for International Peace.
The 1998 test date is a quasi-holiday in Pakistan, and the test site was once declared a national monument, part of the nuclear chest-thumping that, along with political instability, makes U.S. officials as nervous as the actual number of weapons.
In December 2008, Peter Lavoie, the U.S. national intelligence officer for South Asia, told NATO officials that "despite pending economic catastrophe, Pakistan is producing nuclear weapons at a faster rate than any other country in the world," according to a classified State Department cable released late last year by the Internet site WikiLeaks.
Publication of the document so angered Pakistani army chief Gen. Ashfaq Kayani that he told journalists there that the "real aim of U.S. [war] strategy is to de-nuclearize Pakistan," according to local media reports.
In 2009, Congress passed a $7.5 billion aid package for Pakistan with the stipulation that the administration provide regular assessments of whether any of the money "directly or indirectly aided the expansion of Pakistan's nuclear weapons program."
While continuing to produce of weapons-grade uranium at two sites, Pakistan has sharply increased its production of plutonium, allowing it to make lighter warheads for more mobile delivery systems. Its newest missile, the Shaheen II, has a range of 1,500 miles and is about to go into operational deployment, Kristensen said. Pakistan also has developed nuclear-capable land- and air-launched cruise missiles.