(Fortune Magazine) -- This past summer's subprime meltdown involved about $900 billion in now-suspect securitized debt, reckless lending, and consumers who buckled under the weight of loans they couldn't afford. Now another link in the consumer debt chain - credit cards - is starting to show signs of strain. And the fear that the $915 billion in U.S. credit card debt (an uncannily similar figure) may blow up has major financial institutions like Citigroup, American Express, and Bank of America strapping on their Kevlar vests.

Last month, as banks reported their worst quarterly results since 2001, concerns about rising credit card delinquencies began to make their way onto earnings announcements alongside mentions of subprime woes.

First Citigroup (Charts, Fortune 500), reporting a 57% decline in earnings, cited higher consumer credit costs and said it would put aside $2.24 billion in loan-loss reserves to cover future defaults.

In describing the situation to analysts, CFO Gary Crittenden said Citi's credit card holders were beginning to increase the balance on their cards or take cash advances on those cards for the first time - behavior that, in his experience (which includes seven years as CFO of American Express), can translate into future trouble. Citi said the change in loan losses was "inherent in the portfolio but not yet visible in delinquencies."

Then American Express (Charts, Fortune 500) said that it too was seeing "signs of stress" and would boost its loss reserves in its core U.S. card unit by 44%. Capital One (Charts, Fortune 500), Bank of America (Charts, Fortune 500), and Washington Mutual (Charts, Fortune 500) all said they are bracing for a 20% or higher increase in credit card losses over the near and medium term.

So are U.S. credit cards going to be the catalyst for the next seizing up of the global credit markets? It depends on whom you ask.

"We are in a heightened state of alert to monitor a potential domino effect," says Michael Mayo, Deutsche Bank's U.S. banking analyst.

Dennis Moroney, an analyst at TowerGroup, expects credit card delinquencies will rise as consumers, who have until now used home-equity lines of credit to pay off their cards, start ratcheting up higher card debt. When housing prices were rising, it was easy for consumers to tap the escalating values of their homes to keep borrowing. With the home-equity spigot turned off, over-leveraged consumers may have trouble keeping up with payments.

The doomsday scenario would play out something like this: Just like CDOs and other asset-backed securities, credit card debt is sliced, diced, and sold off again as packages of securities. Rising delinquencies would hurt not only the banks involved but the securities backed by the credit card receivables. Those securities would decline in value as consumers defaulted, leading to bank losses as well as portfolio losses in the hedge funds, institutions, and pensions that own the securities. If the damage is widespread enough, it could wreak havoc on the economy much as the subprime crisis has done.

To be sure, there are key differences between the subprime market and the problems brewing with credit cards. The first is that while rising mortgage delinquencies were apparent for months before the subprime market blew up, credit card delinquencies are actually coming off unusually low levels.

"This is absolutely not the next one to blow," says Meredith Whitney, banking analyst at CIBC. Christopher Marshall, CFO of Fifth Third Bancorp in Cincinnati, points out that the U.S. has a long history of credit card securitization, "so it's fairly well understood." The securitization of the subprime sector, by comparison, "got blurry, and people didn't focus on what it meant."

Credit agencies that monitor credit cards in the asset-backed securities market share that confidence. "The performance in the core consumer [asset-backed securities] sectors is expected to deteriorate modestly, but not enough to cause substantial downgrades," says Kevin Duignan, managing director at Fitch.

But credit card debt is different from subprime debt in another way: Unlike mortgages, credit card debt is unsecured, so a default means a total loss. And while missed payments are at a historical low, they show signs of an uptick: The quarterly delinquency rate for Capital One, Washington Mutual, Citigroup, J.P. Morgan Chase, and Bank of America rose an average of 13% in the third quarter, compared with a 2% drop in the previous quarter.

What's more, consumers and the people who market financial services to them may not have learned their lesson. Klaus-Peter M�ller, CEO of Germany's Commerzbank, told Fortune he was stunned on a recent trip to the U.S. to see TV ads still aggressively touting no-questions-asked credit. In Germany he's calling for tighter standards.

"I'm speaking out on the ethical questions about consumer lending," he says.

If there is an international precedent the U.S. should be watching, it's actually that of the U.K. British consumers are just as overstretched as Americans, but since the real estate market there rose faster and fell earlier, they're about 18 months ahead in the credit cycle. Since the last quarter of 2005, credit card delinquencies and charge-off rates in Britain have risen as much as 50%, forcing banks to take huge write-offs.

It's a sign of the times that, according to one survey last month, 6% of British homeowners have been using their credit cards to pay their mortgages. That's suicidal, of course, given that credit card interest rates are more than double even the heftiest mortgage. Keep your fingers crossed that it's not a trend that crosses the Atlantic.