China National Offshore Oil Corp. has filed an application with the US government to review its proposed $15.1 billion acquisition of Nexen, a CNOOC spokesperson said in an email Thursday.

"I can confirm that CNOOC has now filed formally with CFIUS [Committee on Foreign Investment in the United States]," he said.

CNOOC said earlier that it had conferred with CFIUS on July 23 when the transaction was announced and undertook to file voluntarily under the Exon-Florio amendment.

"The parties have stayed in close contact with CFIUS following the announcement, including answering informal questions from the CFIUS staff," the spokesman said in the email.

"While we will not speculate on the review, we have emphasized that the deal poses no threat to US national security ... We have presence in the US and are in compliance with US laws and regulations. Nexen's assets in the US will continue to be subject to all applicable US laws and regulations," he added.

CNOOC on July 23 announced an all-cash offer for Nexen at $27.50 for each outstanding common share in a bid to grow its upstream production and reserves. Nexen has assets globally, including in Canada, the US, the UK and Africa. If approved, this would be China's largest overseas corporate acquisition to date. CNOOC management said August 21 that it welcomed the scrutiny from governments but was confident it would be able to procure all required approvals.

Canadian Industry Minister Christian Paradis said August 29 that he had started a review of the deal, which would take at least 45 days but could be longer.

Nexen, listed in New York and Toronto, has scheduled a shareholder vote for September 20. Completion of the deal is conditional on approval of at least 66% of the votes cast by common shareholders. Nexen's board has unanimously recommended the deal. The two companies hope to close the transaction in the fourth quarter of this year.