SINGAPORE, July 8 (Reuters) - Global index provider S&P Dow Jones Indices SPGI.N warned that Turkey and Indonesia risk a downgrade to frontier from emerging status over market transparency and other issues, echoing similar moves at rival MSCI MSCI.N.

Both markets were added to a watchlist for potential inclusion in next year's market review process, S&P DJI said in a statement late on Tuesday. Nigeria was also on the list for a potential upgrade to frontier from standalone status.

Indonesia has been placed under review by MSCI since January and the risk of a downgrade has tanked an equity market that was once the toast of emerging market fund managers - leaving it down more than 30%, or 35% in dollar terms, for the year so far.