Moody's Investor Services has downgraded the financial strength rating of nine Greek banks as the country's debt crisis takes a hefty toll on their finances.

In a note Friday, Moody's also says that the banks' deposit and debt ratings may be downgraded — these would come alongside Moody's ongoing review of the country's sovereign debt rating.

On Thursday, the agency confirmed that it is awaiting to see details of an EU-IMF rescue package before a possible revision of Greece's credit rating, but that a "multi-notch downgrade" remained likely.

Friday's move reflects "their weakening stand-alone financial strength and the anticipated additional pressures stemming from the country's challenged economic prospects," the agency said.

The banks affected by the downgrade are: National Bank of Greece, EFG Eurobank Ergasias, Alpha Bank, Piraeus Bank, Emporiki Bank of Greece, Agricultural Bank of Greece, General Bank of Greece, Marfin Egnatia Bank and Attica Bank.

S&P has downgraded Greece's sovereign debt rating to speculative grade, sending shockwaves through markets and sending yields on Greek debt higher than Venezuela's.

Moody's did not give any detail about what future action it may take on the rating of Greece's bonds, which it still holds in investment grade territory.