Amid growing US-Pakistan tensions over the detention in Lahore of an American official on possible murder charges, nuclear experts here have reportedly claim that Pakistan may be building a fourth plutonium-producing reactor at the Khushab nuclear complex. The report was carried on Thursday by The Washington Post, which said the move could signal a further escalation in Pakistan's arms race with India. The newspaper said commercial satellite photographs taken last month show major new construction at Khushab, a key nuclear installation that generates plutonium for Pakistan's nuclear arsenal. The new structure is roughly the same size and shape as two plutonium-producing heavy-water reactors located a few hundreds yards away in the heavily guarded compound, according to an analysis by the Institute for Science and International Security, a Washington organization that studies nuclear proliferation. The building "appears to be a fourth reactor" for producing weapons-grade plutonium, according to the ISIS analysis, which gave a copy to The Washington Post. ISIS said the facility would substantially expand Islamabad's nuclear capacity by allowing it to produce "more plutonium for nuclear bombs." Pakistani officials in Washington, asked about a fourth reactor at Khushab, declined to comment, the Post said. A U.S. counterproliferation official who reviewed the images declined to comment on the ISIS analysis but said that U.S. intelligence agencies have been monitoring Khushab for years and are "aware of this facility." The new reactor, if verified, would signal yet another step forward in Pakistan's ambitious effort to modernize and expand its nuclear arsenal. A Washington Post article last month reported that Pakistan's stockpile was estimated to have grown to more than 100 deployed weapons and to have surpassed that of India. The newspaper said the rapid growth of Pakistan's nuclear arsenal has fueled fears of an escalating arms race in one of the world's most troubled regions. "Another reactor just hammers the point that Pakistan is determined to make a lot of plutonium for nuclear weapons, frankly far more than they need or is healthy for the region and the world," said ISIS President David Albright, who co-authored the report with researcher Paul Brannan. The first heavy-water reactor at Khushab became operational in 1996, and a second reactor was inaugurated about a year ago. The two together can generate an estimated 22 kilograms of plutonium a year, enough for up to four bombs. A third reactor is under construction near the second one. Olli Heinonen, former director of safeguards at the International Atomic Energy Agency, said the new reactor was "worrying, given the unstable situation there." "Commissioning of additional plutonium-production reactors and further construction of reprocessing capabilities signify that Pakistan may even be developing second-strike capabilities," said Heinonen, now a senior fellow at Harvard University's Belfer Center for Science and International Affairs.