Moody's Investors Service, the credit rating agency, this morning placed China National Offshore Oil Corporation on review for a possible downgrade following its $18.5bn (€15.3bn) cash bid for US rival Unocal.

CNOOC, which has lined up $19bn in financing for the deal, has an A2 rating at Moody's. The large amount of debt needed to fund the cash deal "will elevate CNOOC's financial leverage to a level that is not consistent with its present A2 rating" Moody's said.