BEIJING/SHANGHAI China's central bank is tightening its scrutiny of bulk dollar purchases by domestic firms, three sources with direct knowledge of the matter said on Monday, at a time when the Chinese currency faces mounting depreciation pressure.

Companies that need to purchase $50 million or more will now need approval from the People's Bank of China (PBOC), which convened a meeting with some commercial banks over the weekend on the matter, the sources said.

The directive is being issued as the Chinese yuan has declined by about 6% against the U.S. dollar so far this year, falling to levels that were last seen during the 2008 global financial crisis. [CNY/]

The PBOC had no immediate comment on plans to increase its scrutiny of dollar purchases when contacted by Reuters.

(Reporting by Beijing and Shanghai Newsroom; Editing by Christian Schmollinger)