Standard & Poor's on Wednesday cut Greece's credit rating to "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.
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.
Read MoreBarroso: Risk of a 'Grexit' is higher today
The ongoing issues have fueled fears that Greece will default on its obligations, prompting a possible "Grexit," or departure from the euro zone.
S&P's rating cut "reflects our view that Greece's solvency hinges increasingly on favorable business, financial and economic conditions."
The moves comes on the heels of a Fitch Ratings downgrade late last month. The agency lowered Greece's long-term foreign and local currency issuer default ratings to "CCC" from "B."
Read MoreFitch downgrades Greece to 'CCC' from 'B'
At a press conference earlier Wednesday, ECB President Mario Draghi said he was "not ready to discuss" a potential Greek default. He noted that there is currently no end date to the emergency liquidity assistance (ELA) the central bank has extended to Greece.
The country sold 812.5 million euros of three-month Treasury bills in the second of two auctions on Wednesday.