Pakistan could be in the advanced stage of building a new reactor at its Chashma nuclear site capable of separating weapons-grade plutonium from spent reactor fuel and it could go into operation soon, a US think tank has said.

The Washington-based Institute for Science and International Security has said that it has obtained commercial satellite imagery of Pakistan's Chashma nuclear industrial park showing what appears to be the construction of a new reactor and its associated facilities, the Daily Times reported from Washington on Saturday.

Although it is unclear whether the facility is operational, the nature and rate of the construction suggests that it may soon start operation, if it has not done so already, ISIS said.

ISIS believes that the imagery raises the question of whether Pakistan intends to operationalise the facility capable of separating weapons-grade plutonium out of spent reactor fuel.

Such a capability, combined with Pakistan's ability to make large quantities of highly enriched uranium for nuclear weapons, would facilitate Islamabad in developing thermonuclear weapons as well as increasing the size of its nuclear arsenal.

The reprocessing facility at Chashma would significantly facilitate Pakistan's plutonium separation capability and lead to an expanded plutonium production capacity, the think tank said.

Given New Delhi's intensified efforts to increase uranium enrichment capabilities at the Rare Materials Plant, the two neigbours could be engaged in expanding the size and quality of their nuclear arsenals, the ISIS report warned.