U.S. chief executive officers' view of the economy reached a four-year high in the second quarter, with more planning to add jobs over the next six months, according to a Business Roundtable survey.

But the leaders of corporate America grew somewhat more cautious in their capital spending plans, suggesting concern about the strength of the nation's economic recovery, the survey released on Wednesday found.

Our CEOs are generally somewhat positive about sales and employment, but we see a slowing of capital spending optimism, said Ivan Seidenberg, CEO of Verizon Communications Inc and chairman of the Roundtable. There is a cautious approach to how much momentum is being developed to expand the economy.

Some 39 percent of corporate chiefs expect to increase their companies' U.S. head count over the rest of this year, more than the 17 percent who plan to cut it.

Companies that do significant volumes of business in the rapidly developing Chinese, Indian and Brazilian markets are showing the most confidence, while the retail sector was more cautious on hiring, Roundtable officials told reporters on a conference call.

The group's index of CEO confidence rose to 94.6 in the second quarter, up from 88.9 three months ago and the highest since the second quarter of 2006, when it stood at 98.6. Any reading above 50 represents a growth forecast.

Seventy-nine percent of U.S. CEOs expect their companies' sales to rise over the next six months, compared with 4 percent who forecast declines.

Forty-three percent plan to increase their U.S. capital spending -- far more than the 7 percent who plan to cut it, but less than the 47 percent who forecast a boost to equipment budgets in March.

Business Roundtable member companies, 106 of which were surveyed between May 24 and June 14, employ some 12 million people and generate almost $6 trillion in collective annual revenue.

Investors will get a more detailed window into corporate outlooks next month, when Alcoa Inc , General Electric Co , JPMorgan Chase & Co and other major companies report quarterly results.

(Reporting by Scott Malone; Editing by Lisa Von Ahn)