Moody's Investors Service on Monday changed its outlook for Germany, the Netherlands and Luxembourg to negative from stable as fallout from Europe's debt crisis cast a shadow over the euro zone's top-rated countries.
Moody's cited 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 troubled states such as Spain and Italy that are struggling to finance their deficits.
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 Aaa-rated Austria and France. 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."
The agency noted that it now has a negative outlook on all the countries "whose balance sheets are expected to bear the main financial burden of support."
"We are in a transitional period, and this transitional period could last for many years, and during this transitional period we do see additional pressure on the strongest nations' balance sheets, which has increased pressure on their credit standing," said Moody's senior credit officer Sarah Carlson.
Spanish bond yields, a gauge of the premium investors demand to lend money to the government, soared on Monday to their highest level since the euro's inception more than a decade ago. Investors fear the country may soon need an emergency rescue.
Investors' rush for safety Monday sent 10-year German government bond yields to record lows and U.S. Treasury bill yields to their lowest since the 1800s.
The German Finance Ministry said the country would remain an anchor of stability in the 17-nation euro zone, adding 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."
Finland escaped a negative outlook partly because of its small and domestically oriented banking system, its fiscally conservative budget policies and its limited trade links with the rest of the euro area, Moody's said.
Moody's also cited Finland's efforts to reduce its exposure to potential loan losses in bailouts of euro area sovereigns. Finland has demanded collateral in exchange for loans to debt-burdened countries in the euro zone.
Rival agency Standard & Poor's maintains a stable outlook for Germany but negative outlooks for Luxembourg, the Netherlands and Finland. All are rated 'AAA'.
Fitch Ratings rates all four AAA and gives them stable outlooks.
Standard & Poor's rates France AA-plus with a negative outlook; Fitch rates France AAA with a negative outlook.
Standard & Poor's rates Austria AA-plus with a negative outlook; Fitch rates Austria AAA with a stable outlook.