LONDON- S&P Global said on Tuesday it will undertake a "case-by-case assessment" of countries seeking debt relief from private creditors using the G20 debt relief plan to determine if they've defaulted on their commercial debt.

Both S&P and Fitch chopped Ethiopia's rating after Addis Ababa signalled it would be the first country with an international government bond, and not already in default, to use a new G20 "Common Framework" plan.

The scheme, which is open to over 70 of the world’s poorest countries, encourages governments to defer or negotiate down their debt to private creditors as part of a wider relief programme.

Any hit to private creditor payments are typically regarded as a default by ratings firms and can trigger widespread financial market and legal problems.

"To the extent that a sovereign seeks debt relief from private creditors (nonofficial), we will undertake a case-by-case assessment to determine whether there has been a default," S&P said in a research note.

(Reporting by Marc Jones and Tom Arnold;) ((Tom.Arnold@thomsonreuters.com; +442075428510; Reuters Messaging: tom.arnold.thomsonreuters.com@reuters.net))