The number of U.S. workers filing new applications for jobless insurance unexpectedly rose last week, but a jump to a record high in a gauge of economic prospects eased concerns about the recovery.

The Labor Department said on Thursday initial claims for state unemployment benefits rose 36,000 to a seasonally adjusted 482,000 last week as a backlog of applications from the holidays were processed.

It was the third straight week that claims rose. Analysts had expected new claims to slip to 440,000.

Separately, a gauge of the economy's prospects rose for the ninth straight month to a record high in December, a private research group said, indicating the recovery was set to pick up.

It looks like the recovery momentum is still very much in place and it suggests that we should have good growth going through the first half of the year, said Alan Gayle, senior investment strategist at Ridgeworth Investments in Richmond, Virginia.

The Conference Board said its index of leading economic indicators rose 1.1 percent to an all-time high of 106.4 last month. Analysts polled by Reuters had forecast the indicator rising by only 0.7 percent.

In another report, the Philadelphia Federal Reserve Bank said its business activity index slipped in January to 15.2 from 22.5 in December, which was a 4-1/2-year high. Analysts polled by Reuters had expected a January reading of 18.0.

U.S. stock indexes fell sharply as shares of major banks and those of natural resource companies sold off, while Treasury debt prices were little changed on the economic reports.

The weekly claims data covers the survey week for the Labor Department's January payrolls report, scheduled for release February 5. Analysts said the backlog suggested a slowdown in the labor market improvement.

That implies that claims in previous weeks were understated so the reduction in new jobless claims between December and January is not as extreme as had been thought. But with lay-offs diminishing, the labor market is still gradually healing, said Pierre Ellis, senior economist at Decision Economics in New York.

The four-week moving average of new claims rose 7,000 to 448,250 last week, snapping a 19-week declining trend. The four-week moving average is viewed as a better measure of underlying trends as it levels out week-to-week volatility.

Labor market weakness remains a major obstacle to recovery, which started in the third quarter of 2009 following the longest and deepest downturn since the 1930s.

The number of workers still collecting benefits after an initial week of aid fell 18,000 to 4.6 million in the week ended January 9, the lowest since January 2009. This was in with market expectations.

So-called continuing claims have declined for five weeks and are below their peak of 6.9 million last June. There were 5.9 million people receiving extended benefits under special programs in the week ended January 2.

The insured unemployment rate, which measures the percentage of the insured labor force that is jobless, was unchanged at 3.5 percent.

(Reporting by Lucia Mutikani and Richard Leong; editing by Dan Grebler)