Juergen Stark told German newspaper Boersen-Zeitung in an interview published Tuesday that the bond buying was necessary to counter malfunctions in the financial markets, but stressed it won't be greatly expanded or continued indefinitely.

The ECB has bought 72.5 billion euros ($95.3 billion) worth of sovereign bonds since May, though prominent members of the bank's governing council - notably Germany's Axel Weber and Italy's Mario Draghi - have publicly expressed unease about the program. Draghi recently warned it threatened the ECB's political independence, while Weber has called for it to be stopped entirely.

´There is no chance we will reach the dimensions seen with other central banks and we won't change the program's current nature,ª Stark was quoted as saying in the interview.

The US. Federal Reserve in November announced that it plans to buy $600 billion in US government bonds over the next six months in an effort to push interest rates even lower to boost the economy by encouraging borrowing and spending. The tactic is called quantitative easing.

Unlike the Federal Reserve, the ECB's bond purchases are not considered to expand the supply of money in the economy because the central bank  "sterilizes" its bond purchases, meaning it reduces liquidity elsewhere by the amount of money it is putting in through its bond buys.

Stark said that each country remains responsible for handling its own debt, and he strongly rejected the idea of the 16-nation eurozone evolving into a transfer union.

"Demands that stronger economies should help weaker countries contradict the monetary union's fundamental principles," Stark was quoted as saying.

Stark said that the European treaties' clause against bailouts must not be questioned and he insisted that the central bank's role was not to finance member countries' sovereign debt.  "It is not our duty to refinance governments," the newspaper quoted him as saying.

The ECB, meanwhile, said in a statement it has extended a liquidity swap arrangement with the Federal Reserve up to August 2011 that allows it to offer dollar-denominated loans to the eurozone's banks.

The decision on extending the liquidity facility, which was set to expire in January, is coordinated with the Bank of Canada, the Bank of England, the Bank of Japan, and the Swiss National Bank, who all have swap arrangements with the Fed, the ECB said.