Sources close to the deal have suggested that CNOOC is likely meet the conditions, which include a number of divestments and a promise to meet US regulators’ demands.

At present Unocal’s public backing remains with its initial suitor Chevron, despite CNOOC’s offer that valued Unocal at $18.5 billion, $2 billion more than Chevron’s bid.

CNOOC has spent the last two weeks in negotiations with Unocal trying to win its backing. It is rumored that the Chinese company could also increase its offer further in an attempt to override Uncoal management and appeal to its shareholders.

Unocal’s request for guarantees comes in response to worries that demands from US authorities may deter CNOOC. The Chinese company already feels aggrieved by the US Congress, which called for a review of its takeover plans after having already voted to block the move in the short term.

Unocal wants assurances that a deal would not fall through after it had rejected Chevron’s interest.