European Union officials on Saturday warned that retailers were reporting problems providing change for the continent's new single currency, the euro, introduced on Jan. 1.

According to the EU's executive commission, the use of the euro had jumped to 55 percent as of late Friday in the 12 countries that adopted it, but some problems have begun to emerge.

"Contrary to expectations, large-denomination national currency notes are in many cases being spent in shops while small sums are being exchanged at banks," the commission said.

Still, the high demand for the new bills has also created minor shortages at banks, whose cash reserves have dwindled quickly, according to the European Central Bank, headquartered in Frankfurt, Germany.

Cash withdrawals from banks have been about four times above normal in the first week of the new currency, and banks have been so preoccupied with the introduction of the new money that they have been slow to return old currencies to central banks for more euros, according to Manfred J. Koerber, spokesman for the ECB.

In order to help banks keep enough euros on hand, the ECB on Friday offered a total of 25 billion euros, or $22.4 billion, in three-day loans to institutions that needed the extra cash.

"There is no shortage," Koerber said. "The euro system has billions of bank notes in stock, and many low denomination notes, but some banks apparently underestimated the demand and did not stock up on enough."

The EU said the practice of dumping large notes in grocery stores and other retailers was causing shortages of small-denomination notes in shops for giving change.

"In a number of extreme cases, supermarkets have been obliged to give change in national currency, and some banks are turning away non-customers," the commission said.

It added that several countries of the 12 EU nations adopting the euro decided to open banks on Saturday to handle the demand for the new currency.

The Netherlands and Greece recorded the highest number of cash payments carried out in euros with 80 percent, while France, Italy and Spain scored the lowest, between 25 and 50 percent, the EU said.