Business activity in the euro zone expanded at an accelerated pace at the start of the second quarter, but companies cut the prices they charged customers, a closely watched business survey showed, a sign that the European Central Bank's worries over low inflation are far from over.

The ECB is prepared to act on interest rates or other stimulus measures if inflation undershoots the central bank's forecasts, ECB board member Ardo Hansson, who heads Estonia's central bank, said in an interview with The Wall Street Journal Wednesday.

Data firm Markit's survey of 5,000 manufacturing and services businesses found that activity expanded again in April, and at the fastest pace since May 2011. The composite Purchasing Managers Index--a gauge of activity across the manufacturing and services sectors--rose to 54.0 from 53.1 in March. A reading above 50 indicates month-to-month expansion in activity.

The business surveys add to recent evidence that the currency area's return to growth, which started in the second quarter of 2013, is gaining some momentum. A survey released by the European Commission on Tuesday found that consumer confidence was at its highest since October 2007 in April, while recent data releases have pointed to rises in retail sales, industrial production, exports and construction.

However, the business surveys carried a warning for the ECB, with manufacturers and services providers reporting that they cut their prices in April at the fastest pace since August 2013.

"There will be growing fears that deflationary pressures are intensifying and that the ECB needs to respond with more than just words to the recent appreciation of the exchange rate," said Chris Williamson, Markit's chief economist.

Mr. Hansson played down the risks of deflation. "It would mean a situation of widespread decreases of prices in several countries and industries. This scenario is difficult to imagine," he said. Still, he said the ECB has a number of options at its disposal--including lower interest rates, long-term loans to banks and asset purchases--if inflation falls short of the central bank's forecasts.

The PMI survey suggests that the currency area's economy is growing too slowly to quickly return the inflation rate to the ECB's target of just below 2%. The ECB's governing council has accepted that inflation will be below target through 2016, but policy makers also believe that the 0.5% annual rate of inflation recorded in March was a low point.

But while April may see a pickup in the rate of inflation, weak growth and subdued wages could keep it low for a sustained period, while an appreciating euro also limits price rises. Low rates of inflation make it more difficult for governments and households to pay down their debts, which would be a particular problem in southern Europe.

The European Union's statistics agency Wednesday said the combined debt of the euro zone's 18 governments rose to 92.6% of gross domestic product in 2013, the highest level since records began in 1995. Governments did manage to cut their new borrowing in 2013, recording a combined budget deficit of 3% of GDP, the smallest since the financial crisis began in 2008.

However, the rise in government debt in the bloc's southern fringe was larger than the average, since Germany managed to cut its government debt to 78.4% of GDP from 81% in 2012.

Greece's government debt soared to 175% of annual economic output from 157.2% in 2012, while Portugal's government debt rose to 129% from 124.1%, and Spain's rose to 93.9% from 86%.

The purchasing manager surveys, which come on the heels of the rise in consumer confidence, challenge assertions made by many economists and the International Monetary Fund that weak consumer-price pressures threaten the euro bloc's yearlong economic recovery.

Although low inflation makes it more difficult to repay large debts, there is an upside, too, that is reflected in recent economic reports. When food and energy prices soften, households have more disposable income to spend.

Low inflation may also reflect improved competitiveness in southern Europe, as businesses keep costs and prices down, economists said.

"It isn't a source of concern," said Jörg Krämer, chief economist at Commerzbank. "You may even say this is good deflation as it goes hand in hand with a recovery in profit margins," he said, noting that asset prices such as equities and bonds in much of Europe have risen, not fallen.

Wednesday's PMI report supports that view. Euro-zone members outside of France and Germany, many of which have seen stagnant or falling consumer prices, saw their sharpest rise in output in more than three years.

Markit's survey found that businesses hired additional workers for only the second month since 2011, and at the fastest pace since September of that year. If sustained, that would start to cut into the euro zone's very high unemployment rate, and eventually translate into higher wages, which should generate inflationary pressures.

Liis Kangsepp contributed to this article

Write to Paul Hannon at paul.hannon@wsj.com and Brian Blackstone at brian.blackstone@wsj.com

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