Chevron and Unocal have now set an August 10 deadline for Chevron’s cash-and-stock offer after the US Securities and Exchange Commission yesterday approved the deal.

The Unocal board says it continues to back the Chevron bid while weighing up the value and risk of CNOOC’s counter-offer.

"Our board will need to understand the risks and value associated with both proposals in order to decide whether to withdraw its current recommendation for the Chevron proposal," Unocal chairman and chief executive Chuck Williamson wrote in a letter to employees that was included in a SEC filing.

But CNOOC executives – who are this week meeting with Unocal in New York – are still optimistic.

"We remain confident in both the superiority of our offer and its chances of succeeding," the Chinese government-owned company said in a statement.

Many analysts believe the CNOOC offer is more financially attractive, but that the Chevron bid offers more certainty.