South Korean group GS Engineering & Construction and Italy's Saipem have won deals to expand a petrochemical complex in Saudi Arabia for oil giant Saudi Aramco and Japan's Sumitomo Chemical.

A Sumitomo spokeswoman confirmed on Tuesday that the partners had picked Saipem and GS to work on the Rabigh II project on the Red Sea coast of the world's largest oil exporter.

GS said that its contract was worth $1.8 billion, while industry sources in Saudi Arabia said that Saipem's deal was worth about $700 million.

Other deals involving Britain's Petrofac and Japan's JGC are close to being signed, sources said, but the Sumitomo spokeswoman declined to comment on contracts yet to be announced.

The Rabigh II project is part of Aramco's plans to diversify its business from crude oil into chemicals, unconventional gas and renewables. Operations are planned to start in the first half of 2016.