Treasury Secretary Timothy Geithner makes a one-day trip to Poland this week for an unprecedented meeting with euro zone finance ministers as growing fears of a potential Greek debt default rip into Europe's banking sector.

The trip comes as a surprise since Geithner returned only on Saturday from a meeting of Group of Seven finance ministers in Marseilles, France, where he said Europe's strongest economies must offer unequivocal backing to the weakest.

Geithner is expected to attend the euro zone meeting on Friday and then return to Washington. The Treasury said on Monday only that he will discuss efforts to boost global recovery and cooperate on financial regulation, but U.S. attention is focused on risks posed by potential European debt contagion.

The danger that a Greek debt default could roil bigger European economies was underlined on Monday as heavily exposed French banks' shares plunged and investor confidence in the euro zone's ability to surmount a sovereign debt crisis ebbed.

Underscoring concerns by the United States about the global economic dangers from Europe's debt troubles, the Treasury Department said Geithner would meet with International Monetary Fund chief Christine Lagarde on Tuesday.

Geithner's trip to Europe marks the first time a U.S. Treasury secretary will attend a meeting of euro zone finance ministers. But it is not the first time he has tried to push Europe into acting more decisively to cope with its debts.

In March, he made a quick one-day trip to Germany just days ahead of a Europe Union summit to meet his counterpart, Wolfgang Schaeuble, and to urge European countries to step up their efforts to handle the crisis.

He held a one-on-one session with Schaeuble again in Marseilles on Friday, but neither side would talk about what was discussed in that session.

On Monday, shares in Societe Generale, BNP Paribas and Credit Agricole slumped more than 10 percent amid expectations of an imminent downgrade by credit ratings agency Moody's due to their exposure to Greek bonds.

The surprise resignation of European Central Bank Chief Economist Juergen Stark on Friday and weekend comments by German politicians suggesting Athens may have to default and be suspended from the euro zone drove the euro to a 10-year low against the yen and a seven-month low against the dollar, though it later recovered some ground.

Europe is not just lurching from one crisis to another. It is lurching into a new one before the previous one is solved, said Makoto Noji, senior strategist at SMBC Nikko Securities.

The storm on Monday forced SocGen, the hardest-hit French lender in recent weeks, to announce further drastic measures it denied only last week were under consideration, speeding up asset disposals and deepening cost cuts to free up 4 billion euros in fresh capital.

The bank's market value has shrunk from 110 billion euros in mid-2007 to just 12 billion on Monday. The bank's chief executive said there were no discussions regarding possible state intervention.

Finance Minister Francois Baroin said French banks were solid enough to withstand any crisis in Greece and Bank of France Governor Christian Noyer rushed out a statement saying French banks were not at risk.

There is no crisis for the banks because those that are currently being hit on the markets have all the necessary means to offer solutions, Baroin told reporters, adding that G7 central banks were committed to providing as much liquidity as banks need.

French banks and insurers are not only the biggest foreign holders of Greek government bonds, both directly and through Greek subsidiaries, but are also major creditors of Italy, which is increasingly in the markets' firing line.

Moody's is expected to downgrade Italy's Aa2 sovereign rating this week, Richard Kelly, head of European rates and FX research at TD Securities said, noting that both Fitch and Standard & Poor's already had lower ratings for Rome.

The Financial Times reported on its website on Monday that Italy has asked China to make significant purchases of Italian debt. The chairman of the China Investment Corp headed a delegation to Rome last week following a visit two weeks ago by Italian officials to Beijing to meet CIC and State Administration of Foreign Exchange officials, the report said.

OMINOUS START

It was an ominous start to a high-stakes week for the euro zone.

The ECB disclosed that it bought another 14 billion euros in euro zone government bonds last week, the biggest amount in three weeks, under a controversial policy to hold down troubled peripheral countries' borrowing costs.

The central bank holds a total of 143 billion euros in Italian, Spanish, Greek, Portuguese and Irish bonds under its securities market program, which drove Stark -- a traditional German central banking hawk -- to resign.

Greece resumed suspended talks with global lenders on a vital 8 billion euro aid installment after announcing a new real estate tax on Sunday to try to plug yet another gap in its 2011 budget deficit. Athens has only a few weeks' cash left.

EU finance ministers are scrambling to settle disputes over a second Greek bailout, including a spat over Finnish demands for collateral, in time for Friday's meeting in Poland.

The rescue package has been put in doubt by Greece's repeated missing of fiscal targets agreed with the EU and the International Monetary Fund, plus uncertainty over the scale of private sector participation in a bond swap and debt rollover.

Germany tried to douse the market impact of a string of weekend comments and media leaks suggesting Berlin is now assuming that Greece will default and working to seclude Athens from the rest of the euro zone.

Vice-Chancellor Philipp Roesler, who is economics minister and leader of Berlin's increasingly euroskeptic junior coalition party, the Free Democrats, said there could no longer be any taboos to stabilize the euro.

That includes, if necessary, an orderly bankruptcy of Greece, if the required instruments are available, he wrote in an article in Die Welt newspaper.

However, an economics ministry spokesman said on Monday no such instruments were currently available, and a government spokesman insisted there was strong agreement between Roesler and Chancellor Angela Merkel on the euro zone debt crisis.

We want to stabilize the whole euro zone with all member states, government spokesman Steffen Seibert said.

Asked about talk of a suspension, expulsion or voluntary departure of Greece from the euro zone, he said: The legal position anyway stands in the way of such steps.

Seibert added that if Athens did not meet its fiscal commitments to the EU, ECB and IMF, that would automatically lead to nonpayment of the next tranche of aid.

Greece's deputy finance minister said the government had cash to operate until next month, highlighting the urgent need for the next emergency loan.

Greek power workers threatened to sabotage the new property tax announced by the government on Sunday as a last-ditch effort to please foreign creditors. Authorities plan to collect the tax through electricity bills to ensure swift payment.

(Additional reporting by Anirban Nag in London, Elena Berton and Jean-Baptiste Vey in Paris, Harry Papachristou in Athens and Brian Rohan in Berlin; Writing by Paul Taylor and Glenn Somerville; Editing by Dan Grebler)