Rating agency Standard & Poor's on Tuesday raised Greece's sovereign credit rating to B-minus with a stable outlook from selective default, citing its European partners' efforts to keep the country part of the euro.

"The upgrade reflects our view of the strong determination of European Economic and Monetary Union (eurozone) member states to preserve Greek membership in the eurozone," S&P said.

"The outlook on the long-term rating is stable, balancing our view of the government's commitment to a fiscal and structural adjustment against the economic and political challenges of doing so," the agency added.

Euro zone partners and the International Monetary Fund have agreed to unlock 49.1 billion euros in aid by the end of March. The decision to release the long-delayed installment came after Athens passed austerity measures and completed a debt buyback.