Singapore, July 2 (Reuters) - At least five supertankers carrying a total of 10 million barrels of Saudi crude loaded at Ras Tanura have left the Strait of Hormuz, according to trade sources and shipping data, as Saudi Aramco switched to a spot pricing system to accelerate sales in Asia.

Saudi Aramco, the giant Saudi oil company, resumed loading operations at Ras Tanura, the world's largest oil port, on Friday after a nearly four-month hiatus. The company is ramping up loadings and shipments to Asia, adding to the immediate oversupply that has driven Brent crude prices down to around $70 a barrel from nearly $120 in March following the preliminary peace agreement between the United States and Iran.

In addition to using a fleet of tankers belonging to Bahri to deliver shipments, the world's largest oil exporter offered crude oil to its Asian customers on a spot basis to attract demand as competition intensifies among suppliers, according to several trading sources who declined to be identified due to the sensitivity of the matter.

Aramco declined to comment on the matter.

Aramco typically sells oil through long-term contracts at official selling prices (OSPs) set by the producer each month. However, its OSPs for July-loading cargoes to Asia, set in early June, come with premiums of between $6 and $10 per barrel, while other Middle Eastern oil sales for July-August have fallen to reduced levels following progress in talks between the US and Iran, with most refiners having already purchased enough oil to last until August.

One source said that six million barrels of crude oil to be loaded in July were offered to Aramco's usual Asian customers. Another source said the prices were "very attractive" to Chinese buyers.

Traders expect Aramco to sharply reduce its official selling prices for August.