Wealthy investors have grown more pessimistic about the economy this summer, with many citing crises in the Middle East and Ukraine as potential hazards.

The August Millionaire Investor Confidence Index from Spectrem Group shows that millionaire confidence remains roughly flat for the month and is still well below the spike in confidence in June. While millionaires are generally more bullish than less-wealthy households, the July and August levels ended a steady four-month rise in the spring.

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The main reason: trouble in the Middle East and Ukraine. When asked about news most affecting their economic outlook, the largest number of millionaires (49 percent) cited "international problems." That was followed by "the political environment" and "unemployment," which ranked far behind at 8 percent each. News about the overall economy was cited by only 6 percent of millionaires.

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"From Ukraine to Israel to Iraq, flare ups in international crises are having a direct impact on affluent investors economic outlook," the report said.

The overseas jitters have made millionaires a little more wary of stocks. The number of millionaires planning to invest in stocks fell 3 percent. Millionaires planning to invest in bonds fell 1.7 percent.

—By CNBC's Robert Frank

