Fitch Ratings warned it would treat a voluntary rollover of Greece's sovereign bonds in any rescue package as a default and would cut the credit rating, keeping pressure on European policymakers who intend to outline a new plan by mid July.
Fractious euro zone finance ministers are trying to patch together a second aid package for Greece, with more official loans and, for the first time, a contribution by private investors of Greek government bonds.
"The essence of the problem ... is that Greece needs new money," Andrew Colquhoun, head of Asia-Pacific sovereign ratings with Fitch, said at a conference in Singapore.
"Fitch would regard such a debt exchange or voluntary debt rollover as a default event and would lead to the assignment of a default rating to Greece," he said.
Fitch said it also saw risks of a debt default in the United States, whose top-rated bonds may suffer if the country doesn't lift its fiscal borrowing ceiling, as several developed countries struggle to deal with increased debt built up during the global financial crisis.
Greece has rattled financial markets as the Athens government and European policymakers try to avert the first debt default by a euro zone country.
Fitch's comments on Greece caused the euro to cut some gains on the day in Asian trade, although the reiteration of risks surrounding U.S. Treasuries kept the single currency from dropping more against the dollar.
After two days of crisis talks in Brussels, euro zone finance ministers issued Athens an ultimatum, saying the government, parliament and broader society had until July 3 to approve new steps in order to get the next installment of the 110 billion euros in European Union and International Monetary Fund aid agreed in May 2010.
Greek Prime Minister George Papandreou faces a confidence vote on Tuesday, a hurdle he must clear to push forward with securing support for the austerity package that is the condition for the installment of 12 billion euros.
Euro zone ministers said they were also ready to put together a second loan package for Greece of around 120 billion euros ($168 billion), to support the country longer term.
It would include investors making voluntary purchases of new Greek bonds as existing ones mature. Fitch estimates that Greece has a funding gap of around 80 billion euros to mid 2014.
A month ago Fitch downgraded Greece's credit rating three notches to B plus and warned it could cut the rating further into junk territory. At the time, the rating agency said an extension of the maturity of existing bonds would be considered a default.
Fitch is not alone in warning that the EU's ideas may be propelling Greece towards a debt default.
Standard & Poor's cut Greece's rating to CCC from B on June 13, and warned that any attempt to restructure the country's debt would be considered a default. It reiterated that point in comments to German newspaper Die Welt, due to be published on Tuesday.
Moody's has a Caa1 rating on Greece's sovereign debt, which implies a 50 percent chance of a default within three to five years.
As European policymakers scramble to avoid a Greek debt default, derivatives traders called on them not to deliberately avoid triggering a credit event in the credit default swap market, otherwise investors may view the instruments as ineffective gauges of default risk.
It could force investors to stop using CDS as a way to hedge their risk, they said.
The euro area debt crisis has already prompted Greece, Ireland and Portugal to seek financial rescue. Concern that Greece may be heading for a debt default is growing, leading to some tightening of credit markets and conference calls on Sunday and Monday by G7 finance ministers.
Washington says the Greek crisis could pose a risk to the fragile global recovery following the financial crisis.
The United States, alongside the likes of Japan and Britain, has its own fiscal debt worries.
Colquhoun reiterated that Fitch would place the U.S. sovereign rating on watch negative if Congress did not raise the federal government's borrowing ceiling by Aug. 2. The U.S. Treasury Department says Congress must raise the $14.3 trillion debt ceiling by then to avoid a potential default.
If the U.S. government misses an Aug. 15 coupon payment, Fitch would place the rating on restricted default. Members of the U.S. Republican party have flirted with the idea of a U.S. default as an acceptable price to pay if it forces the White House to cut spending sharply in negotiations over raising the national debt limit.
Still, Colquhoun said it believed the debt ceiling would be raised and a default would be avoided.
Fitch made similar comments earlier this month and Moody's and S&P have issued warnings along the same lines. But Fitch was the first major ratings agency to say U.S. Treasury securities could be downgraded, even for a short period.
U.S. lawmakers working to rein in rising debt said on Monday that they will have to make substantial progress this week to ensure the country retains its top-notch credit rating.
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