Tax rebate checks and robust exports helped the U.S. economy grow at a faster-than-expected rate in the second quarter, the government reported Thursday. But some economists warned that those two factors can't prop growth up for long.

U.S. gross domestic product increased at a 3.3 percent annual pace in the April-June quarter, according to the Commerce Department. That was the best showing since the third quarter of 2007, beating the government's earlier estimates of a 1.9 percent growth rate and topping economists' forecasts of 2.7 percent.

The GDP measures the value of all the goods and services produced by the U.S. and is considered the best gauge of the nation's economic well-being.

The government attributed the increase to accelerating exports and a drop in imports, a rise in spending by consumers and by state and local governments, and signs of improvement in the housing sector.

Exports grew at a 13.2 percent rate in the quarter, more than double the first-quarter rate.

Consumer spending rose 1.7 percent, the biggest increase in nearly a year, as government rebate checks of up to $600 per person sent shoppers scurrying to malls and big-box retailers.

The report gave a boost to Wall Street, where the Dow Jones industrials jumped 212.67 points to close at 11,715.18. Lower oil prices also helped the markets.

Can it last?

However, some economists questioned whether those factors can sustain economic growth through the second half of the year and into 2009.

"We can't be overly upbeat about this particular report," said Bernard Baumohl, chief global economist at the Economic Outlook Group in Princeton, N.J.

The last of the stimulus checks went out early in July, Baumohl noted, and their boost to consumer spending is largely over.

Global weakening

In addition, the global economy is starting to slow, which could dampen the appetite for U.S. goods abroad. That could be especially true in Europe, where weakening economic conditions are helping pump up the dollar in relation to the euro. A stronger dollar makes U.S. exports more expensive and less competitive.

In addition, while the Labor Department reported that first-time jobless claims declined last week, the number remained above the 400,000 level that economists consider the dividing line between a growing or shrinking economy. Employers have clamped down on hiring, driving the nation's unemployment rate up to 5.7 percent in July, a four-year high.