WASHINGTON (AFP) - 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. The Institute for Science and International Security, a private US group which is critical of nuclear weapons, said Tuesday it observed progress at Pakistans 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 wouldnt 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 India carried out similar atom bomb tests. Pakistans 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 miniaturised 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 thats what Pakistan wants, Brannan said.