Pakistan appears to have stepped up construction of a new atomic reactor that could help the country produce easier-to-deliver nuclear weapons, a US research institute said.

Pakistan's nuclear arsenal is one of the most sensitive topics for the United States as it tries to improve relations with its frontline partner in the campaign against Islamic extremism.

The Institute for Science and International Security, a private US group which is critical of nuclear weapons, said Tuesday it observed progress at Pakistan's tightly guarded Khushab site which is key to plutonium production.

In a September satellite image of the site in Punjab province, the institute said it observed a completed row of mechanical draft cooling towers at a third reactor, where construction began in 2006.

It marks a faster pace than for the second reactor, where such towers appeared after six years of construction, it said.

"Based on what I see in the image, it wouldn't surprise me if they started it up in 2011," said Paul Brannan, a senior analyst at the institute.

The institute noticed steam from the second reactor in a December 31 image, indicating it was running. It did not see steam in the latest image, but said reactors were not operated continuously during early phases and that weather conditions may have reduced visibility.

Pakistan declared itself a nuclear weapons state in 1998, days after its historic rival India carried out similar atom bomb tests. Pakistan's nuclear arsenal originally was based on highly enriched uranium.

Western analysts believe that China initially assisted Pakistan in developing Khushab nuclear site to produce plutonium, which can be miniaturized for cruise missiles -- presumably aimed at India.

"Plutonium bombs give the ability to make smaller, lighter or more powerful weapons, and also more deliverable weapons, and I suspect that's what Pakistan wants," Brannan said.

Pakistan, which experts estimate now has up to 100 nuclear weapons, has been adamant that its nuclear weapons are in safe hands and President Barack Obama has publicly concurred.

But the United States hinted at its frustration on Tuesday at the United Nations, where Pakistan has blocked a resumption of negotiations for an agenda in global nuclear disarmament talks.

Pakistan opposes a proposed Fissile Material Cutoff Treaty, which would limit access to highly enriched uranium and plutonium used to make nuclear weapons.

Pakistan believes the treaty would lock in a nuclear imbalance in favor of India, with which it has fought three full-fledged wars since independence in 1947.

Rose Gottemoeller, the US assistant secretary of state in charge of arms control, warned "our patience will not last forever."

"I have to tell you that I expressed some disappointment at the fact that the conference on disarmament over the last years has been less energetic in terms of pursuing its overall agenda," she told reporters.

Pakistan's Prime Minister Yousuf Raza Gilani, visiting Washington in April for a nuclear security summit, said his country had tight control over its weapons and urged the United States to offer civilian nuclear cooperation of the type it has with India.

Joseph Cirincione, president of the Ploughshares Fund which supports a nuclear weapons-free world, said that the current safety of Pakistan's arsenal was not the issue.

"It's the security of the government that worries me. If the government falls that's when the nightmare comes," Cirincione said.

"American politicians and policymakers live in a constant state of denial about Pakistan. They see a mess and then they look away and pretend it's all going to get better somehow," he said.

The United States is the only nation to have dropped an atomic bomb in combat but Obama has set a goal of an eventual world without nuclear weapons.