A flagging world economy took a toll on much of Corporate America in the third quarter, leading the likes of eBay Inc, American Express, IBM and Textron Inc to miss Wall Street's sales targets or warn that spending was slowing into the holidays.

Those misses sparked concerns among investors that corporate America's year-long streak of profit growth could be nearing an end as CEOs run out of costs to cut and customers are increasingly wary about spending.

A major worry is the risk of the year-end fiscal cliff - $600 billion of spending cuts and higher taxes that could take effect at the end of the year if lawmakers in Washington fail to agree on a plan to shrink the federal deficit.

A majority - 54.3 percent - of the 70 companies in the widely watched Standard & Poor's 500 Index that have reported results so far have missed analysts' revenue forecasts, according to Thomson Reuters I/B/E/S.

The list of companies' reasons for weak performance has expanded, with some citing a decline in demand in the United States - until recently a more reliable source of growth - as well as in Europe, which is mired in a debt crisis.

American Express warned that its cardmembers' spending was starting to wane. After nine quarters of double-digit growth, spending among its mostly affluent customer base grew just 8 percent in the last quarter.

The online marketplace eBay beat estimates, but analysts said the magnitude was unimpressive and the holiday outlook cautious as price competition increases among retailers.

International Business Machines Corp, the world's largest technology services company, missed analysts' sales forecasts for a fifth consecutive time with a 5 percent drop in the third quarter, as corporate customers in the United States and Canada cut their spending on equipment and services.

"When I look at our skew of business in the quarter, through the first two months, our revenue was fairly consistent with our second-quarter performance," IBM's Chief Financial Officer Mark Loughridge told investors after the company reported earnings late Tuesday. "The third month of the quarter was more challenging."

Chipmaker Intel Corp also said lower corporate spending hurt its revenue, which was down 5 percent for the quarter. It set a fourth-quarter sales target that was below analysts' forecasts.

While troubles in the tech sector reflect in part a lack of corporate confidence, with companies holding back on committing to major new investments, soft drink makers PepsiCo and Coca-Cola Co also reported weaker-than-expected revenue.

PepsiCo said on Wednesday that its sales in the Americas were down about 3 percent in the quarter, which partly reflected its decision to stop selling some juices and bottled water packages after "a hell of a price war," CEO Indra Nooyi said.

The company, which also makes Frito-Lay snacks and Tropicana orange juice, also noted that the strong dollar hurt its results by decreasing the value of sales made outside the United States.

Many major U.S. companies with international operations have cited the strong dollar as a drag on growth this year.

Drugmaker Abbott Laboratories also reported weaker-than-expected revenue after its sales slipped 0.4 percent to $9.77 billion in the quarter.

No. 2 oilfield services company Halliburton Co said slowing U.S. drilling took a toll on its profit in the quarter.

'CRACK IN THE ARMOR'

Of the S&P 500 companies that have reported so far, 64.3 percent have beaten Wall Street's lowered expectations. At the start of the third quarter on July 1, analysts expected those 500 companies to collectively increase profits by 3.1 percent, a forecast that was cut to a drop of 2.1 percent by Oct 1.

"Earnings expectations have been lowered so far that it hasn't been hard to beat them," said Keith Springer, president of Capital Financial Advisory Services in Sacramento, California. "Once revenue starts missing and you can't cut costs anymore, I think this is the crack in the armor."

While the broad Standard & Poor's 500 Index rose moderately on Wednesday, more revenue misses could take the wind out of a long climb in stocks this year. The S&P 500 has gained about 16 percent so far this year.

"Once we start to see earnings miss - when expectations have been pushed down and they still miss - that is when you'll see the market start to fall apart," Springer said.

Not all observers took as grim a view of the situation.

"The U.S. economy is in better shape today than it has been for some years," said Charles King, president of tech consultancy Pund-IT. "It is in a turnaround even if it is not as strong as we'd like it to be."

SETTING THE STAGE FOR 2013

With about two-and-a-half months left in 2012, investors are starting to turn their attention to next year, and analysts are pushing CEOs to offer some hint of how they expect their companies to perform in 2013.

After Textron reported weaker-than-expected sales and earnings, it noted that orders for its Cessna corporate jets had been very weak in July and August, though they had recovered somewhat in September.

Management said orders would need to hold at September's rate for the rest of the year for Cessna to meet its sales goal.

Textron CEO Scott Donnelly declined on Wednesday to offer a sales forecast for 2013, but allowed that production rates - the number of aircraft the company makes - would likely be "flattish," when compared with 2012 levels.

Even companies that beat Wall Street's earnings targets, such as insurer UnitedHealth Group Inc, cautioned investors not to set their hopes too high for 2013.

"We continue to be cautious about 2013 earnings performance," UnitedHealth CEO Stephen Hemsley told investors in a Tuesday conference call. "We expect to grow revenues and earnings per share in 2013, but we view the current consensus level as a considerable challenge.

CEOs may be wise to reel in expectations for 2013 as it would be difficult to push profit margins much higher, given the slowdown in revenue growth, said Peter Klein, senior portfolio manager at Fifth Third Asset Management in Cleveland, Ohio.

"Margins are pretty rich," Klein said. "If you have slower revenues and you've cut as much (in) expenses as you can, there's not much more you can do" to grow earnings."

(Reporting by Scott Malone in Boston; Additional reporting by Martinne Geller and Nicola Leske in New York; Editing by Jan Paschal)