Germany has begun to throw its weight behind measures to guard Europe’s financial system from a possible Greek or other government debt default, as Chancellor Angela Merkel said Wednesday that she would support a continent-wide plan to pump more capital into banks if they need aid.

Merkel’s comments add momentum to an effort urged by U.S., International Monetary Fund and other officials for Europe to ensure that its banking system can survive if Greece needs to restructure its $300 billion in outstanding bonds.

Once considered unthinkable, that possibility — a sovereign default inside a major world currency zone — is now a central facet of the planning among European and IMF officials, who are trying to both keep Greece afloat and buffer the broader regional economy against the chance that they may not succeed.

A Greek default would have unpredictable consequences for the world economy. The turmoil in Europe has been cited by Treasury Secretary Timothy F. Geithner as a chief concern for the U.S. economy. Trade and the health of U.S. banks with large European ties could all suffer if Europe slips into a new recession, as some analysts project.

But the shock would be minimized if it was clear that European banks maintained large enough stores of capital — cash and other highly liquid assets — to absorb losses on their Greek bonds and any aftershocks that might follow a default.

The effort to contain Greece’s financial problems led to a nationwide strike on Wednesday, as thousands of Greek workers protested the government’s austerity measures and paralyzed the country’s transportation system.

Some banks have already begun writing down the value of their Greek holdings, and a July plan for the country envisions cutting the value of Greece’s debt by about 21 percent.

Merkel, the IMF and others now acknowledge that the July plan may not provide enough help for a Greek economy that is slipping further into recession, amid sometimes intense riots and a growing sense of social upheaval.

Alongside extra billions of dollars in public aid from the IMF and Europe, private banks may have to take an even larger hit to stabilize Greece’s finances.

“If there is a common view that banks aren’t sufficiently capitalized for the current market condition,” a system to aid them should be set up, Merkel said in Brussels on Wednesday after meeting with European Commission President Jose Manuel Barroso. She urged that the plan be developed quickly.

The IMF on Wednesday urged action as well. IMF Europe division head Antonio Borges said at a news conference in Brussels that a collapse of confidence in Europe’s economy, bank health and crisis management was undercutting growth.

A key step in restoring trust, he said, would be a quick and regionwide capital infusion.

The IMF contends that banks in the euro area face potential losses of $300 billion on their holdings of Greek and other government debt that is now considered a “high credit risk.”

Those losses may never materialize, and European bankers insist that their institutions are on the whole financially sound.

But the uncertainty surrounding the issue has taken a high toll, with share prices in bank stocks plummeting in recent weeks and officials stumbling for new ideas to restore trust in the system.

Borges on Wednesday suggested the IMF might even start buying European government bonds to help stabilize markets — a step that would require fundamental changes in the way the agency operates.

He quickly retracted the notion, noting that the IMF is — and plans to remain — in the business of lending directly to governments, not intervening in securities markets.

European officials are divided over whether new capital requirements should be issued by national regulators on a bank-by-bank basis or imposed regionally as a way to undergird the whole system at once. They also differ on whether additional capital should come from private investors, banks’ profits or governments. Added capital also could come from the new European Financial Stability Facility that is being set up in response to the debt crisis.

The discussion took on added import this week when France and Belgium had to pledge new support for the Brussels-based Dexia bank.

The troubled Franco-Belgian institution has made extensive loans to governments suffering financial troubles — including Greece, Italy, Spain, Portugal and Ireland.

International investors, including U.S. money market mutual funds, have pulled out of the European bank market over concerns about the health of the system. Banks in Europe have stopped lending to one another over concerns about each institution’s ability to repay even the routine short-term loans that keep the financial system lubricated with cash. That tightening in credit is constricting the region’s already slow growth.

In Britain on Wednesday, Prime Minister David Cameron told a Conservative Party convention that he thought the looming economic threat was as dangerous now as it was three years ago. He said that he did not plan to slacken his country’s austerity measures even though economic growth has been weaker than expected.

The British Office for National Statistics said Wednesday that second-quarter growth in the country had been nearly flat. The economies of other major countries, including France and Germany, have also been sputtering in recent months.

Britain is not part of the 17-nation euro currency zone, having chosen to maintain the pound sterling.

“The threat to the world economy — and to Britain — is as serious today as it was in 2008 when world recession loomed,” Cameron said.

Schneider reported from Washington.