Standard & Poor's on Wednesday cut Greece's credit rating from "CCC+" from "B-" with a negative outlook, saying it expected Greece's debt to be "unsustainable." The ratings agency cited the potential for dissolving liquidity in the Greek government, banks and economy. S&P wrote that outlook for full-year economic growth looked "highly uncertain." "Without deep economic reform or further relief, we expect Greece's debt and other financial commitments will be unsustainable," S&P wrote in a release quoted by CNBC.

Greece is currently weighed down with debt payments from its 240 billion euro rescue loan from creditors including the International Monetary Fund, European Central Bank and European Commission. As part of a February extension of Greece's bailout, the cash-strapped nation needs to submit economic reforms for creditor approval.