The refinery is designed to process 400,000 barrels-per-day (bpd) of crude, sources said. It is part of Saudi Arabia's plans to boost domestic refining capacity from the current level of 2.1 million bpd.

The due date to bid for the front-end engineering and design (FEED) and project management services (PMS) was Dec. 21 but has already been extended to Dec. 25 on the request of bidders, sources said. Bidders are now asking for more time.

Eight international engineering firms have been prequalified to bid: US firms KBR, Foster Wheeler, Mustang Engineering, Fluor, CB&I Lummus, Jacobs Engineering, along with France's Technip and Australia's WorleyParsons.

"All (companies) are requesting an extension, further time," said one source.

"Most likely (bidding) would be extended until the second week of January," a second source said.

Saudi Arabia had hoped the refinery would be built and owned entirely by the private sector, a first in the world's top oil exporter.

In January, the government handed over the delayed project to Aramco. The refinery is to be built in Jazan, far from the Kingdom's producing oil fields, as part of a wider development plan to revive the impoverished southern region.

The plant is to produce around 75,000 bpd of gasoline, 100,000-160,000 bpd of ultra-low-sulfur diesel and 160,000-220,000 bpd of fuel oil depending on the crude mix processed, Aramco said on its website.

It would be integrated with a power and water facility. A marine terminal would also be built.

Minister of Petroleum and Mineral Resources Ali Al-Naimi was quoted as saying in July the plant would be completed by 2014.

But two industry sources said on Wednesday it was expected to come online by 2016.