"The continued rapid escalation of the euro area sovereign and banking credit crisis is threatening the credit standing of all European sovereigns," the agency said in a new special comment.
Moody's said that "in the absence of policy measures that stabilise market conditions over the short term, or those conditions stabilising for any other reason, credit risk will continue to rise."
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Greece, Ireland and Portugal all suffered rating downgrades that accelerated unsustainable rises in their borrowing costs over the past two years.
Spain and Italy, which has opened its books to international auditors, have also come under pressure in recent days.
France recently announced deep budget cuts in a bid to retain its top Triple-A rating status.
Economists say it is struggling to hold onto the top ranking it shares with the stronger eurozone economies of Germany, the Netherlands, Austria, Finland and Luxembourg.
Moody's noted that political uncertainties in Greece and Italy and the worsening of the economic outlook across the euro area had given rise to "the likelihood of even more negative scenarios.
"The probability of multiple defaults ... by euro area countries is no longer negligible," the agency warned.
"In Moody's view, the longer the liquidity crisis continues, the more rapidly the probability of defaults will continue to rise."
The agency also cautioned that a series of defaults would also "significantly increase" the likelihood of one or more members not simply defaulting, but leaving the eurozone.
"Moody's believes that any multiple-exit scenario -- in other words, a fragmentation of the euro -- would have negative repercussions for the credit standing of all euro area and EU sovereigns," the agency added.
