A Beijing-based energy giant which will take over a Canadian firm in what is reportedly China's biggest-ever foreign investment has said local employees will benefit from the deal.

Canadian Prime Minister Stephen Harper on Friday announced regulatory approval for the $15.1 billion takeover by the China National Offshore Oil Corporation (CNOOC) of oil and gas company Nexen.

The approval was given despite opposition in Canada to the takeover.

"We believe the transaction will provide opportunities for Nexen employees and CNOOC," said Li Fanrong, chief executive of the Chinese firm.

"We are also very pleased that the Ministry of Industry approved a transaction which will bring about 'mutual benefit' to Canada," he added in a statement posted on the company's website on Saturday.

CNOOC chairman Wang Yilin said he was "delighted" the acquisition was approved.

The deal comes seven years after political panic about China's thirst for global energy assets scuppered a massive bid to take over California's Unocal.

It also comes two months after a US congressional committee said Chinese companies Huawei and ZTE should be excluded from government contracts because their equipment could be used for spying.

The CNOOC deal is China's largest foreign investment and its largest energy deal, according to data firm Dealogic.

Calgary-based Nexen produces the equivalent of around 213,000 barrels of oil a day, with concessions in Canada's oil sands, Britain's North Sea, Nigeria, the Gulf of Mexico and Colombia.