Analysts say the move is a blatant attempt to dictate the outcome of the Chevron-CNOOC battle for control of Unocal and has little to do improving US security. Rather, it underlines increasing US concerns over the growing global economic influence, and military might, of China.

The proposed review – now included in a long-awaited energy bill due to be passed later this week - would likely mean a four-month delay before the US government's committee on foreign investments in America could even begin reviewing CNOOC's bid for Unocal.

Analysts also say that delay will probably kill any CNOOC deal. The Unocal board has already indicated it preferred the lower US$17 billion bid from fellow Californian rival Chevron because of the political uncertainties surrounding the offer from CNOOC, whose parent company is 71% owned by the Chinese government.

Critics of the CNOOC bid have linked it more general complaints, such as the loss of manufacturing jobs in the US, the rampant counterfeiting of American brands in China, the growing US trade deficit and regulations restricting American investment, along with worries about China's military strength.

Unocal shareholders are due to vote on the revised US$17 billion Chevron offer, which the Californian company last week increased from its original US$16.6 billion, on August 10.