OTTAWA -- Canada on Friday approved two major takeovers of energy firms by the Chinese state-owned giant CNOOC and Malaysia's national oil company Petronas -- deals worth a more than an estimated US$20 billion in total.

Prime Minister Stephen Harper announced regulatory approval for CNOOC's $15.1 billion takeover of oil and gas company Nexen, and Petronas' offer for Canadian gas producer Progress Energy Resources, estimated at $5.5 billion.

However, he cautioned that Ottawa would henceforth block attempts by foreign state-owned companies to buy controlling stakes in Canada's lucrative oil sands.

"In light of growing trends and following the decisions made today, the government of Canada has determined that foreign state control of oil sands development has reached the point at which further such foreign state control would not be of net benefit to Canada," Harper told a press conference.

"The larger purposes of state-owned enterprises may go well beyond the commercial objectives of privately owned companies," he said.

"It is not an outcome any responsible government of Canada could ever allow to happen. We certainly will not."