ATHENS — Greece’s plan to buy back some of its bonds from private investors ‘‘must succeed’’ as it is a vital part of efforts to reduce the country’s excessive debt, Finance Minister Yannis Stournaras said Wednesday.

The bond buyback is part of measures agreed in Brussels this week that included the release of $57 billion in critical rescue loans from the International Monetary Fund and the other 16 European Union countries that use the euro. The bulk of those funds are to be released by Dec. 13, with more than $12.9 billion to go to internal financing.