Evidence has emerged this week suggesting that Pakistan may have accelerated its covert nuclear weapons development programme and rendered operational a nuclear reactor structure located near a heavy water reactor, in a complex that is likely geared toward the production of plutonium.

High-resolution satellite imagery dated January 15, 2015, shows that external construction of the Khushab complex’s fourth reactor is complete and it has “become operational.”

If, as the evidence suggests, Pakistan is accelerating its nuclear weapons programme, it may heighten tensions with New Delhi, where the subject is likely to come up when Prime Minister Narendra Modi meets U.S. President Barack Obama during Republic Day celebrations next week.

In a report that included the satellite photographs by Digital Globe, Washington think-tank Institute for Science and International Security (ISIS) said that the assessment of reactor completeness and operation was “based on the presence of a very specific signature: steam is venting from the reactor’s cooling system.”

The Khushab reactor complex was originally constructed and became operational in the 1990s, at that time comprising primarily of a heavy water production plant and an estimated 50 megawatt-thermal (MWth) heavy water reactor.

Following the nuclear tests in India and Pakistan in 1998, Islamabad initiated the construction of a second heavy water reactor between the year 2000 and 2002, ISIS notes, a third one in 2006, and a fourth one in 2011.

ISIS, which has closely tracked the progress on the construction of the reactor complex, noted that a January 2011 image showed the building, similar in layout to the second and third reactors at the same site, early in its construction but by April 2011, the frame of the reactor building and the main reactor hall were visible.

However, images from November 1, 2013, clearly show that in addition to the near-completion of the fourth reactor’s external structure, the reactor stack and four of the six auxiliary buildings also present in reactors two and three “appear complete.”

Further, ISIS noted, two support buildings located immediately to the west of the reactor building were still under construction but the initial section of the cooling tower row is also visible and “appears about 30 per cent complete.”

Despite this apparent progress ISIS said, “Work on the fourth reactor has proceeded at a much slower pace than previously predicted,” and while December 2014 and January 2015 imagery also shows that the third Khushab reactor is operational, “no steam is visible at reactor 4.”

Meanwhile, there is additional concern, even if it is too early to make a definitive assessment, that some part of the grounds of the complex have been cleared to make room for a fifth reactor, one that could be located closer to a nearby river.

In its 2013 report ISIS said that the increased capability at the Khushab site “would allow Pakistan to build a larger number of “miniaturised plutonium-based nuclear weapons in order to complement its existing arsenal of highly-enriched uranium weapons.”

In recent years concerns have mounted that Pakistan may be considering the use of theatre or tactical nuclear weapons as a response to any conventional military aggression from neighbours, although Islamabad has reportedly not provided any public information about the latest reactor construction.

Speaking to The Hindu earlier, when prior developments at Khushab had been similarly picked up by satellite imagery, Paul Brannan, Senior Analyst at ISIS, said that given that “only few nations in the world have nuclear production capability [this development at Khushab], leading to a possible doubling of plutonium output in Pakistan, is significant.”