WASHINGTON (Reuters) – The US economy likely grew at its fastest pace in nearly two years at the end of 2011, but a rebuilding of stocks by businesses and weak exports could be early warning signs of a slowdown in early 2012.

US gross domestic product is expected to have expanded at a 3.0 percent annual rate in the fourth quarter, according to a Reuters poll, which would be a sharp acceleration from the 1.8 percent clip of the prior three months and the quickest pace since the second quarter of 2010.

''The economy ended 2011 on a fairly positive note, but the composition of growth in the last quarter is not favorable for growth early this year,'' said Ryan Sweet, a senior economist at Moody's Analytics in West Chester, Pennsylvania.

Growth in the fourth quarter is seen getting a temporary boost from a mild start to winter, which bolstered construction, and the building of business inventories after they declined in the third quarter for the first time since late 2009.

A burst of consumer spending on autos is also expected to have helped. The Japanese earthquake and tsunami early last year caused supply disruptions, leaving showrooms bereft of popular models.

As those factors fade, the recovery is expected to lose a step.

Expectations of soft growth going forward led the Federal Reserve to say on Wednesday it expected to keep interest rates at rock bottom levels at least through late 2014.

Fed Chairman Ben Bernanke said the central bank, which forecast growth this year in a 2.2 percent to 2.7 percent range, was mulling further asset purchases to speed up the recovery.

The Fed warned that the economy still faced big risks, a suggestion that Europe's debt crisis could still hit hard.

''The Fed is attempting to shield the economy from a potentially more severe recession in Europe,'' said Sweet.

''Even though the economy improved last quarter there are a number of headwinds and a lot of uncertainty surrounding Europe, emerging markets and also US fiscal policy.''

The main wild card in the GDP report is inventories, since the government has only two months of data. Nonetheless, economists believe inventories rose as businesses prepared to meet stronger consumer demand.

Consumer spending, which accounts for about 70 percent of US economic activity, is expected to have stepped up from the third-quarter's 1.7 percent pace, largely driven by pent-up demand for motor vehicles.

But sluggish income growth amid an 8.5 percent unemployment rate, which has prompted households to tap savings and credit cards to fund their purchases, is expected to weigh on consumers as the new year unfolds.

''Though the unemployment rate has improved, the jobs market remains a major challenge. Part of the decline in the unemployment rate is due to the fact that ... people have stopped looking for work,'' said Adolfo Laurenti, deputy chief economist at Mesirow Financial in Chicago.

''The high level of people out of the workforce and underemployed people show there isn't really much income generation to contribute to a better spending pattern.''

About 23.7 million Americans are either out of work or underemployed.