The U.S. stock market is one of the most expensive in the world and may be prone to bubbles, Nobel Prize-winning economist Robert Shiller warned.

Measuring valuations with his cyclically adjusted price-to-earnings index (which is price divided by 10-year average earnings) the Yale University professor told CNBC that his "CAPE index ... is higher than it's been, except 1929, 2000 and 2007," ominous years for the market.

"The public [also] worries that the stock market is overpriced," he said. "Their confidence in the level in the market is at its lowest since 2000."

He warned that bubbles can be formed "when people think it's time to get in, even though it's overpriced."

However, a MarketWatch survey of stock strategists and portfolio managers found that while no one believes stocks are cheap, too much is being made of high short-term valuations.

MarketWatch reported that just because stocks are pricey doesn’t mean they can’t get more expensive. It is just a matter of how much more, said Dan Greenhaus, chief strategist at BTIG.

“Further, valuations are high but given the level of interest and inflation rates, one could rationalize that level,” he told MarketWatch.

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