"I am not yet sounding the alarm. But in many countries stock exchanges are at a high level and prices have risen sharply in some property markets," Shiller told Sunday's Der Spiegel magazine. "That could end badly," he said.

"I am most worried about the boom in the U.S. stock market. Also because our economy is still weak and vulnerable," he said, describing the financial and technology sectors as overvalued.

He had also looked at "drastically" higher house prices in Rio de Janeiro and Sao Paulo in Brazil in the last five years.

"There, I felt a bit like in the United States of 2004," he said, adding he was hearing arguments about investment opportunities and a growing middle class that he had heard in the United States around the year 2000.

The collapse of the U.S. housing market helped trigger the 2008-2009 global financial crisis.

"Bubbles look like this. And the world is still very vulnerable to a bubble," he said.

http://ca.news.yahoo.com/nobel-prize-economist-warns-u-stock-market-bubble-145958281--sector.html