State-run Saudi Aramco has extended the bidding deadline for a clean fuels project at its biggest oil refinery in Ras Tanura, sources familiar with the plan said.

The potential $2 billion scheme to remove sulphur from refined products is part of a drive to meet stricter environmental standards.

It has already had at least three bidding rounds.

The deadline is now July 17, one of the sources said, adding it had been extended from a May closing date because companies needed more time to prepare bids.

In an emailed response to Reuters, Saudi Aramco said it "does not comment on rumour or speculation".

Saudi Aramco has been revamping its refineries to produce cleaner fuels.

The Ras Tanura clean fuels project, which includes a naphtha hydrotreater, was to be part of a second phase of upgrades and was originally due to go on stream in 2016.

Saudi Aramco CEO Amin Nasser said in March it was working on both its upstream and downstream projects, including the clean fuels developments, without any cancellations.

Global oil majors and Saudi Arabia have been paring back investments to cope with lower oil prices. The kingdom is the world's largest exporter of crude oil.-Reuters