Companies from China and Iran will this weekend sign the first commercial contracts to redesign an Iranian nuclear plant as part of an international deal reached in 2015 over Iran's nuclear program, China's Foreign Ministry said on Thursday.

The fate of the Arak reactor in central Iran was one of the toughest sticking points in the long nuclear negotiations that led to the agreement, signed by Iran with the United States, Britain, France, China, Russia and Germany.

In the redesign, the heavy water reactor will be reconfigured so it cannot yield fissile plutonium usable in a nuclear bomb.