The Institute for Science and International Security, a Washington-based nuclear watchdog, has obtained satellite images showing that a row of cooling towers at Pakistan's secret Khushab-III reactor has been completed. This suggests the plant could begin operation within months, allowing Pakistan substantially to increase its stockpiles of weapons-grade plutonium.

Last year, Barack Obama, US president, called for "a new treaty that verifiably ends the production of fissile materials". In response, the Conference on Disarmament, a 64-nation coalition that negotiated the 1992 Chemical Weapons convention and the 1996 Comprehensive Nuclear Test Ban Treaty, agreed to negotiate a Fissile Materials Cut-off Treaty, intended to cap production of weapons-grade enriched uranium and most forms of plutonium.

But Pakistan, which is deepening its nuclear ties to China, has blocked the Conference on Disarmament from starting discussions, saying a cut-off would hurt its national security interests. Ashley Tellis, a scholar at the Carnegie Endowment for International Peace, said: "Pakistan thinks its going to be forced to cap its fissile material stocks and wants to make sure it has as much as it can get before then." The country's position has frustrated many states. Rose Gottmeiler, the US Assistant Secretary of State for Arms Control, recently warned that her country's "patience is running out".

Khushab-III is the latest in a series of reactors built to feed Pakistan's nuclear weapons programme. Khushab-II, located next to its new sister plant, became operational in February. The plutonium produced at the complex allows for the construction of small but lethal weapons: a single kilogram can produce an explosion equal to 20,000 tons of conventional explosives.