Saudi Aramco and U.S. group Dow Chemical extended to Monday a deadline for companies to qualify for bidding to build a giant petrochemical project, industry sources said.

The plant, which would be one of the largest in the world, is part of the Saudi state firm's ambition to develop its petrochemical industry as the kingdom seeks to move away from over-reliance on oil revenues.

One source from an international engineering firm took the extension of the deadline, previously set for July 6, as a signal the project was definitely going ahead in spite of earlier doubts about the cost.

"Definitely, the project is moving ... There is a whole team working on it," the source said.

Originally the plant was to be built at Ras Tanura, home to the world's biggest offshore oil facility, and would have been integrated with an expansion of the refinery there.

Neither Aramco or Dow Chemical, the largest foreign investor in Saudi Arabia's energy sector, has officially said there has been a change of plan.

However, Saudi Arabia Oil Minister Ali al-Naimi was quoted as saying last month the kingdom's decision to relocate the project to Jubail had saved billions of dollars.

Industry sources said the project's cost, initially expected to be more than $20 billion, could be around $5 billion less.

As well as saving money that would have been required for reclaiming land at Ras Tanura, the new location would benefit from infrastructure already in place, as well as synergies with an existing refinery at Jubail and a new one under construction, Naimi said.

Industry sources have also said the move meant the complex - originally expected to produce 8 million tons per year of petrochemicals from 35 process units - would be smaller, with at least five fewer processing units.

Bidding to build it is expected to start in the fourth quarter of the year, the sources said.