Moody's Investors Service launched a sweeping review of its ratings on bonds backed by commercial-real-estate mortgages, responding to tumbling property values and soaring defaults.

The review of $302.6bn of debt accounts for more than half the commercial-mortgage-backed securities -- or CMBS -- rated by Moody's, a subsidiary of Moody's Corp. At least $80 billion of that debt used to finance office towers, shopping malls, hotels and other commercial property may be downgraded, Moody's said.

Wall Street Journal