FRANKFURT — Mario Draghi, president of the European Central Bank, warned Thursday that the structure of the euro zone had become “unsustainable” and criticized political leaders he said have been slow to respond to the sovereign debt crisis.


“The configuration we had for 10 years, which was considered sustainable, has been shown now to be unsustainable unless further steps are undertaken,” Mr. Draghi told a committee of the European Parliament in Brussels.


In what may have been his bluntest criticism of political leaders since taking office in November, Mr. Draghi said that half-measures and delays by political leaders have made the euro zone crisis worse. He said they need to decide what kind of euro zone they want.


“The next step is for our leaders to clarify what is the vision for a certain number of years from now,” he said. “The sooner this has been specified, the better. Dispel this fog.”


Mr. Draghi, appearing before the Economic and Monetary Affairs Committee of the European Parliament, also delivered a bit of good news. He said that the E.C.B. has resumed normal lending to Greece’s four largest banks after they received fresh capital, easing fears of bank failures in the country that is at the center of the euro zone crisis.


As deposits flow out of Greece and Spain and fears of a bank run rise, Mr. Draghi underscored the E.C.B.’s determination to make sure that solvent banks do not fail because they are temporarily short of funds.


“The E.C.B. will continue lending to solvent banks,” Mr. Draghi told the committee. “We will avoid bank runs by solvent banks.”


But Mr. Draghi warned that the E.C.B.’s ability to prop up the euro zone banking system was close to its limits, and that it was up to political leaders to do more.


Mr. Draghi’s comments reflected the E.C.B.’s frustration with political leaders who have often tried to shift responsibility for fighting the crisis to the central bank, which is relatively immune to voter wrath.


While the E.C.B. can make sure that banks have enough money to operate from day to day, Mr. Draghi said, it cannot replenish depleted capital reserves. Nor can it solve the problem of excessive government debt or the inability of countries like Greece to compete in international export markets, he said.


“Can the E.C.B. fill the vacuum left by lack of euro area governance?” he asked. “The answer is no.”


A similarly stern warning came from Olli Rehn, vice president of the European Commission, who is responsible for economic and monetary affairs and the euro. Euro zone members must find ways to prevent the crisis from spreading from one country to another and hold down borrowing costs “if we want to avoid a disintegration of the euro zone,” Mr. Rehn said in Brussels, according to a text of a speech to a conference in Brussels.


Mr. Draghi called on the European Union to establish a deposit insurance fund to reassure citizens that money in European banks is safe, as well as to regulate big banks at a European rather than national level.


“Greater centralization of supervision is essential,” Mr. Draghi said. “Bankia and other cases show this,” he added, referring to the troubled Spanish bank that is at the center of renewed euro zone tensions.


He cited Bankia as an example of how dithering by political leaders has made the crisis worse. It took too long for Spanish authorities to deal with the problems at Bankia, he said.


“Everybody winds up doing the right thing at the highest possible cost and price,” Mr. Draghi said.


Earlier this month, the E.C.B. cut Greek banks off from normal lines of credit because they had exhausted their capital reserves and were no longer eligible to receive the credit under the central bank’s rules. The Greek banks continued to receive money from the European system of central banks, but through a separate program known as Emergency Liquidity Assistance.


While the switch from one source of credit to another was largely technical, it confirmed that Greek banks were severely undercapitalized and came during a period of heightened tensions when almost any negative news can rattle markets.


On Monday, after a delay of several weeks, the Greek government provided the country’s four largest commercial banks with €18 billion, or $22.5 billion, in capital from the European Union, clearing the way for them to qualify again for E.C.B. loans.


The banks are National Bank of Greece, Alpha Bank, EFG Eurobank and Piraeus Bank.