Spain's Telefonica and No. 2 Chinese mobile operator China Unicom will deepen their strategic alliance by buying an additional $500 million stake in each other, the Spanish company said on Sunday.

The two companies, which together have more than 550 million clients in Europe, Asia and Latin America, are exploring joint investments in parts of Asia and Africa where they are not already present.

After the new purchase, Telefonica's stake in China Unicom will rise to 9.7 percent, while China Unicom's ownership of Telefonica will reach a total of 1.37 percent, Telefonica said in a statement to the securities regulator in Madrid.

China Unicom, which will purchase 21.8 million shares of Telefonica in the coming days at an agreed on price of 17.16 euros per share, will get a seat on Telefonica's board.

Telefonica will buy $500 million worth of China Unicom shares from third parties over a nine-month period.

Telefonica shares closed on Friday at 18.37 euros per share.