SHANGHAI, Dec 6 (Reuters) - China's major state-owned banks were selling U.S. dollars for yuan in the onshore spot foreign exchange market on Wednesday, three sources with knowledge of the matter said.

Such dollar selling was meant to support the Chinese currency as the local unit faced renewed depreciation pressure after rating agency Moody's on Tuesday cut its outlook on China's government credit ratings to negative from stable.

State banks in China usually trade on behalf of the country's central bank in the currency market, but they could also trade on their own behalf.

(Reporting by Shanghai Newsroom; Editing by Tom Hogue)