German Chancellor Angela Merkel plans to announce Germany's ban on short-selling on Wednesday, a coalition source told Reuters on Tuesday.

U.S. stocks tumbled amid reports of the planned move.

Another source said earlier that Germany would ban naked short-selling on certain stocks and euro government bonds from midnight.

"From midnight today there will be a ban on naked short selling of certain stocks and euro government bonds," a source told Reuters. No further details were immediately available.

Economy Minister Rainer Bruederele told Reuters that it was possible the short-selling ban would be quickly enacted.

Naked short-selling is perceived to be at the heart of the troubles around Greece obtaining reasonable rates in the credit markets during its financial crisis, experts have said. Naked short selling differs from short selling in that the sold shares aren't borrowed in advance, experts say.

No other details were immediately available.