The US House of Representatives has voted for a measure opposing CNOOC's bid for the US oil exploration group Unocal.

Congress voted 398 to 15 for a non-binding resolution that called for the US government to block the $18.5 billion bid by China National Offshore Oil Corporation (CNOOC).

The resolution said oil and gas are "strategic assets" and that a Chinese state-run firm's takeover "would threaten to impair the national security of the United States."

The move came days after CNOOC indicated it was launching a cash bid for Unocal, a major US oil and gas exploration firm, seeking to top a merger offer from US giant Chevron, which reached a merger deal in April worth an estimated $16.3 billion.