WASHINGTON (Reuters) - The U.S. government should block a bid by China's state oil company CNOOC for Canadian oil company Nexen until China's government provides "tangible, enforceable commitments" to ensure U.S. companies receive fair treatment when making investments in China, Democratic U.S. Senator Charles Schumer said.

Schumer, the Senate's No. 3 Democrat and a frequent critic of China's trade and currency policy, said the deal gives the U.S. government a good opportunity to hold China accountable for promises it has made to improve market access for U.S. companies.

"I respectfully urge you, in your capacity as chairman of the Committee on Foreign Investment in the United States (CFIUS), to withhold approval of this transaction until China's government has made tangible, enforceable commitments to ensure U.S. companies reciprocal treatment," Schumer said in a draft letter to Treasury Secretary Timothy Geithner, to be sent on Friday.

(Reporting by Roberta Rampton; Editing by Eric Beech)