While economists quibble, the world's richest man has decided: the United States is already in recession. So Warren Buffett tells German magazine Der Spiegel in an interview to be published on Monday.

"It is perhaps not a recession in the way that economists would understand it, but people are already feeling the effects and it will be deeper and longer than people think," Buffett said on a visit to Frankfurt.

Buffett, the 77-year-old chief of the Berkshire Hathaway holding company, blamed financial institutions for introducing instruments "they can no longer control" and said the "genie can no longer be put back in the bottle."

Buffett, who overtook Bill Gates this year as the world's richest man, said he believed the financial markets should be more tightly regulated.

According to the Forbes annual billionaire's list published in March, Buffett saw his wealth jump from 52 billion dollars last year to 62 billion, pushing Microsoft co-founder Gates into third position after 13 years at the top.

US economic growth has slowed dramatically in recent months and a growing number of economists believe the world's largest economy will experience a recession during 2008 amid a housing slump and related credit crunch.