Moody's Investors Service on Monday cut its outlook for Germany's creditworthiness to "negative" from "stable," but confirmed the country's triple-A rating.
Moody's also cut its outlook for the Netherlands and Luxembourg, which also have triple-A ratings, to negative from stable. Finland kept its stable outlook.
The ratings agency said the move was due to growing uncertainty caused by the debt crisis.
It said there was an increased chance that Greece could leave the euro zone, which "would set off a chain of financial sector shocks ... that policymakers could only contain at a very high cost."
It also warned that Germany and other countries rated AAA might have to increase support for ailing countries such as Spain and Italy. The burden of that support would fall most heavily on the euro zone's top-rated states, it said.
The agency affirmed Finland's AAA rating and stable outlook, but said all four countries were adversely affected by uncertainty about the outcome of the euro area debt crisis.
Moody's said it would also weigh the euro zone developments' impact on Austria and France, which it still rates AAA. Those countries' outlooks were lowered to negative in February, and Moody's now expects, by the end of the third quarter, to "review whether their current rating outlooks remain appropriate or whether more extensive rating reviews are warranted."
France and Austria lost their triple-A ratings from Standard & Poor's, another American ratings agency, in January.
'Germany Is in Very Solid Economic and Financial Situation'
The German Finance Ministry played down the Moody's announcement, saying on Monday night that Germany would remain an anchor of stability in euro zone and that Moody's was focusing on short-term risks.
"By means of its solid economic and financial policy, Germany will retain its 'safe haven' status and continue to play its role as the anchor in the euro zone responsibly," the ministry said in a statement.
It added: "Germany continues to find itself in a very solid economic and financial situation."
Financial markets fell sharply on Monday on fears that the crisis was escalating again with reports that the International Monetary Fund and Germany as well as other top creditors were not prepared to extend fresh aid to Greece beyond the €130 billion ($158 billion) rescue package agreed earlier this year.
The German economics minister, Philipp Rösler, added to uncertainty on Sunday by saying he was skeptical that Greece's reform efforts would succeed.
The German DAX share index fell by as much as 3.5 percent on Monday and closed down 3.2 percent.