China's state-owned oil company CNOOC has formally asked the U.S. government to review its $15.1-billion (U.S.) takeover bid for Canada's Nexen Inc. for any national security concerns, a CNOOC spokesman said on Wednesday.

About 10 per cent of the Canadian oil producer's assets are in the United States. The CNOOC spokesman reiterated that the deal does not threaten U.S. security.

CNOOC declined to comment on the precise date when the company filed its paperwork with the Committee on Foreign Investment in the United States (CFIUS).

The confidential panel, which is chaired by Treasury Secretary Timothy Geithner and includes the secretaries of state and defence, has up to 75 days to decide whether to impose any conditions on the deal from the day the paperwork was filed.

The Treasury Department had no comment. Under U.S. law, the Treasury cannot comment on CFIUS reviews.

A handful of U.S. lawmakers had asked Mr. Geithner to review the deal with one urging the panel to block the deal in order to extract trade and investment concessions from the Chinese government.

The CNOOC spokesman said the company has stayed in close contact with the government panel since the deal was announced and has been answering informal questions from CFIUS staff.

Last Wednesday, CNOOC asked the Canadian government to review its Nexen acquisition. The Canadian government also has up to 75 days to probe the merger and decide whether it will benefit Canada.