U.S. wholesale inventories barely rose in December, the latest suggestion that economic growth in the fourth quarter was even slower than initially thought.

Still, the economic outlook remains buoyant, with other data on Tuesday showing job openings in December vaulting to their highest level since early 2001.

"The labor market is shifting into overdrive with jobs out there for nearly everyone. The litmus test for whether or not the expansion is on track and a solid one is always jobs creation," said Chris Rupkey, chief financial economist at MUFG Union Bank in New York.

Wholesale inventories gained 0.1 percent as lower crude oil prices depressed the value of petroleum stocks in December, the Commerce Department said. It reported an advance of 0.8 percent in November.

Inventories are a key component of gross domestic product changes. The component that goes into the calculation of GDP - wholesale stocks excluding autos - nudged up 0.1 percent.

That, together with data last week showing a 0.3 percent fall in manufacturing inventories in December, suggested the boost to GDP growth from restocking in the fourth quarter was probably not as large as initially thought.

The U.S. government estimated last month that inventories added 0.8 percentage point to the economy's annualized 2.6 percent growth pace in the fourth quarter.

Economists said the data in hand suggested that contribution could be cut by at least five-tenths of a percentage point, lowering the fourth-quarter GDP growth rate to 1.7 percent.

Prior to Tuesday's inventories data, a larger-than-anticipated trade deficit in December had led economists to expect that GDP growth could be lowered by as much as 0.4 percentage point in the fourth quarter.

STRONG JOBS MARKET

The government will publish its revised GDP growth estimate later this month. The moderate pace of inventory accumulation, however, could be a boost to first-quarter growth, which also continues to be supported by a strengthening labor market.

In a separate report on Tuesday, the Labor Department said job openings surged to 5.03 million in December, the highest level since January 2001, from 4.85 million in November.

Hiring jumped to a seven-year high, while the number of job seekers for every open position - a key measure of labor market slack - fell to 1.73 in December, the lowest since 2007.

Government data last Friday showed the economy added more than a million jobs over the past three months, a feat last achieved in late 1997.

Economists said the labor market was tightening enough for the Federal Reserve to start raising interest rates by mid-year. It has kept its key short-term interest rate near zero since December 2008.

"Between the solid growth in labor demand and what we view as little remaining labor market slack, we expect that a modest pick-up in wage growth will give Fed policymakers the confidence in the inflation outlook needed to move toward an initial rate hike by mid-year," said Jesse Hurwitz, an economist at Barclays in New York.

The increase in the number of people receiving a paycheck, combined with the stimulus from lower gasoline prices, should boost consumer spending and blunt the blow on the economy from weaker exports and business investment.

In a third report on Tuesday, the National Federation of Independent Business said its Small Business Optimism Index fell 2.5 points to 97.9 last month, reversing December's gains, which had taken the index over 100 for the first time in eight years.

"We are not too worried by the fall since many of the key sub-indices held up well. That drop back is a bit hard to square with the strength of consumer confidence," said Paul Ashworth, chief U.S. economist at Capital Economics in Toronto.