NEW YORK, June 26 (Reuters) - Standard & Poor's on Friday said it has refined its criteria for rating some commercial mortgage-backed securities to ensure top-rated deals can withstand extreme economic downturns without defaulting.

The new criteria will require greater enhancements for an AAA rating based on property value declines of 40 to 50 percent under conditions of extreme stress, such as during the Great Depression, S&P said in a statement.

"This represents a major recalibration of our CMBS criteria and is intended to make U.S. CMBS ratings more comparable with ratings in other sectors, such as U.S. corporates, U.S. municipals, sovereigns, and other areas of structured finance," S&P said.

The revised criteria's main focus is on scenarios that are considered highly unlikely but are still consistent with achieving the highest rating, S&P said.

The revisions affect the ratings on 3,563 tranches from 217 conduit/fusion CMBS transactions, S&P said. Of these tranches, ratings on 1,979 had already been placed on review for possible downgrade. The remaining ratings were placed on review for downgrade on Friday.

Projected impacts are most significant on recent-vintage (2005-2008) CMBS, S&P said. Transactions from the 2007 vintage will likely experience the most significant rating changes. (Reporting by Dena Aubin; Editing by Leslie Adler)