SHANGHAI, July 21  -- Shanghai authorities on Monday vowed transparency and timely disclosure in its ongoing probe into the alleged use of stale meat in products supplied to fast food chains including McDonald's and KFC.

Law enforcement officials had suspended production at a suburban plant of Shanghai Husi Food Co., Ltd, a unit of U.S. food supplier OSI Group, after a news program aired by local television on Sunday claimed the company was using stale meat to make products, the municipal government said in a statement.

McDonald's has sealed more than 4,500 boxes of beef, pork and chicken supplied by Husi, while Pizza Hut has halted use of more than 500 boxes of beef products, according to the statement.

The local food and drug regulator will conduct checks in meat-processing factories and at fast food chains.

The exposure is another blow to Western fast food chains like McDonald's and KFC, which have been implicated in a number of food safety scandals involving suppliers over the years.

OSI Group said it is assisting regulators with the investigation and apologized to consumers.