CNOOC, China's top offshore oil and gas producer, has hired Goldman Sachs to advise it on a bid for a stake in Kosmos Energy, an Africa-focused oil and gas company, according to people familiar with the matter.

The hiring, understood to have been sealed in recent days, underlines the Chinese group's seriousness about joining a likely bidding war for Kosmos, which has been valued at $3bn-$6bn by analysts. Goldman declined to comment.