Saudi Aramco has started pumping oil from its giant Manifa field in the Gulf, allowing the world's biggest oil exporter to relax production from some of its mature fields onshore.

The offshore oilfield - made up of rigs on manmade islands linked by 41 kilometres of causeways and bridges over the Gulf - is expected to produce 500,000 barrels per day (bpd) by July and 900,000 bpd by the end of 2014.

"It really opens a new page in terms of overcoming various hurdles and complexities most notably through human and technological innovation," Aramco's chief executive Khalid Al-Falih said in a statement.

Manifa, discovered back in the 1950s, will not boost the kingdom's sustainable production capacity beyond its stated 12.5 million bpd because Aramco plans to ease production at some of its mature reservoirs to increase their ultimate recovery rates in the long term, Falih has previously said.

Manifa will help the state-run company maintain export levels without running its older fields so hard, while supplying a new 400,000-bpd refinery run by Aramco and France's Total at Jubail on the OPEC heavyweight's east coast with Arabian heavy crude.

"You will not see a change in export and production capacities," a Saudi industry source said, adding "Production capacity will continue as 12.5 (million bpd) for Saudi Arabia."

The Manifa field will also be used to supply crude to an Aramco joint-venture refinery with China's Sinopec in Yanbu, on the Red Sea coast.