Chinese oil giant CNOOC has asked a key US panel to review its proposed $18.5bn (£10bn) merger with oil and gas producer Unocal.
US politicians query state-owned CNOOC's motives
The bid is politically controversial and the firm hopes a review by the Committee on Foreign Investments in the US (CFIUS) will advance its cause.
CNOOC, which has always said it wanted a review, said its voluntary request was aimed at clearing up doubts.
Some US politicians oppose CNOOC's bid on national security grounds.
On Thursday, the US House of Representatives voted to block the Bush administration from backing the deal. CNOOC's request is understood to have been made before the vote.
CNOOC's bid trumps a $16.3bn cash and share offer for Unocal tabled by fellow US oil group Chevron but some US politicians have opposed the Chinese offer.
Political concerns centre both on America's $160bn trade deficit with China, whose economy is surging ahead, and Beijing's emerging political and military power.
CNOOC will now hope that the Committee on Foreign Investments in the US (CFIUS) - which considers proposed acquisitions of US firms by foreigners where national security might be a concern - will dispel misgivings about the deal.
The Xinhua news agency quoted a CNOOC spokesman on Saturday as saying: "It's a voluntary notice from CNOOC Ltd. We hope the review will be put onto formal proceedings of CFIUS as soon as possible as we are sure that the transaction is purely commercial."
"This filing gives CNOOC the opportunity to comply with all US rules and regulations in an open and transparent manner, and to fully discuss our proposal," the Xinhua news agency quoted Yang Hua, CNOOC's chief financial officer, as saying.