General Motors Co and PSA Peugeot Citroen will announce their joint purchasing agreement soon, GM Chief Executive Dan Akerson said on Sunday.

"There will be announcements later this month, early next month on that," he told reporters at an event ahead of the Sao Paulo auto show.

Last month, French and Brazilian union officials said PSA Peugeot Citroen and GM were working on plans to build small cars together in Brazil. Under the tentative plan, the allied automakers would pool production of two new models for Peugeot Citroen and two for GM. No decision had been made on a production site, union officials said.

Akerson called bringing GM's Cadillac luxury brand to South America in the next five years a "possibility."

Akerson also said the U.S. automaker remains "cautiously optimistic" about vehicle demand in China, where it is the market leader. He said GM has gained market share, which he said the company now estimates to be about 15 percent.

(Reporting by Ben Klayman; Writing by Asher Levine)