Saudi Arabia put a second 4% chunk of shares of the Aramco oil giant, worth tens of billions of dollars, under the control of the country's sovereign wealth fund, the state-run Saudi Press Agency (SPA) reported Sunday.

The move, part of a campaign to open up the oil-rich kingdom's economy, follows a transfer of 4% of Aramco shares last year, estimated to be worth $80 billion.

Authorities did not give a value for the latest shares, which have been moved from the world's biggest oil exporter to Sanabil Investments, a firm controlled by the Public Investment Fund (PIF) led by Crown Prince Mohammed bin Salman, SPA said.

The crown prince, Saudi Arabia's de facto ruler, billed the move as part of a push "to boost and diversify the national economy and expand investment opportunities," according to the report.

The shares of the primarily state-owned oil giant moved last year went directly to the PIF, one of the world's biggest sovereign wealth funds with more than $620 billion in assets.

The crown prince said the latest transfer would "solidify PIF's strong financial position."

He also stressed that the state "will remain Saudi Aramco's largest shareholder following the transfer, with total ownership of (90.18%) of the company's shares," the report said.