The US House of Representatives has voted to block the Bush administration from backing a Chinese takeover attempt for US oil firm Unocal.
The vote could pour hot water on CNOOC's bid
The Republican-led house voted against any backing for the $18.5bn (£10bn) bid put forward by China's CNOOC oil firm.
But the move needs approval from the Senate, which has not considered such a proposal.
CNOOC's bid trumps a $16.3bn cash and share offer tabled by fellow US oil group Chevron.
On Thursday the US government gave the go-ahead to Chevron's takeover of its US rival, the ninth biggest oil firm in the US.
The approval turned up the heat on CNOOC's offer, leaving the group just six weeks to persuade Unocal to take up its cash offer.
US politicians have opposed CNOOC's bid on national security grounds.
The concerns centre on both America's $160bn trade deficit with China, whose economy is surging ahead, and Beijing's emerging political and military power.
News of the vote followed a trading update from Unocal.
The oil and gas producer said it expected its profits to meet, if not exceed, even the most optimistic forecasts thanks to the recent surge in oil prices to record highs.