US lawmakers have called for close examination of the $18.5bn (£9.8bn) unsolicited bid for American oil firm Unocal from Chinese rival CNOOC.
US legislators are fearful about energy security
In a letter to the Treasury Department, 41 Democrat and Republican politicians voiced concerns that a takeover by CNOOC may compromise national security.
Worries have been amplified by recent trade rows with China, a ballooning trade deficit and record oil prices.
CNOOC's offer came after Unocal had accepted a buyout by US firm Chevron.
In a letter to Treasury Secretary John Snow obtained by the Reuters news agency, the Congressmen warned that US firms would find it increasingly difficult to compete with Chinese oil companies for "scarce energy resources".
"We know that you understand well the critical importance of ensuring US energy security and the critical need secure the future of availability of energy resources for American consumers," the lawmakers wrote.
Mr Snow oversees the Committee on Foreign Investment in the United States, which could highlight security risks and move to block the deal.
CNOOC has moved quickly to try and reassure the US that it is interested only in securing future energy supplies.
The Chinese firm has said it is ready to hold talks with takeover target Unocal and the US government about its bid.
CNOOC has said it may be willing to sell off some of Unocal's businesses and keep the management of some of others in US hands.
Unocal has said it will consider the Chinese bid, but added that it was committed to Chevron's lower bid.
Chevron has moved its timetable for the takeover forward to August in an effort outmanoeuvre CNOOC.
However, the Chinese firm is optimistic its all-cash proposal will win out over Chevron's cash and share offer.