US tax authorities say they are looking into more than 100 accounts held by Americans in tax haven Liechtenstein.
The tax investigation is the biggest in German history
At least nine nations are now examining tax evasion related to the tiny Alpine principality after a massive investigation by German prosecutors.
The probes are believed to be based on data from an informant, a former employee of Liechtenstein's LGT Bank.
The US announcement came as Germany widened its investigation to a second bank in Liechtenstein.
'No hiding place'
German prosecutors said the homes and offices of 150 suspects had been raided and trusts worth 200m euros (£150m) were being examined.
"It should be clear from recent events that there is no safe hiding place for the proceeds of tax avoidance and evasion," said Linda Stiff, the acting commissioner of the US Internal Revenue Service, according to a statement quoted by the AFP news agency.
"Anyone with hidden income and gains would be well-advised to make a prompt and complete disclosure to the Internal Revenue Service," Ms Stiff said.
The head of the Swedish tax office in Stockholm said that France, Italy, Spain, Canada, the United States, Australia and New Zealand were also examining information on accounts in Liechtenstein.
The UK's tax authority on Sunday said that it had paid an informant for data regarding British citizens with accounts in Liechtenstein.
Shares in Swiss private bank Vontobel dropped sharply on Tuesday after a newspaper said its Liechtenstein unit was the unnamed target of the German prosecutors.
Vontobel rejected the reports and said none of its client data was being examined in connection with the affair.
Germany launched the inquiry after reportedly paying 5m euros in January 2006 for a list of wealthy Germans with money stashed in Liechtenstein.
Berlin said on Monday that it would share information it had obtained with other countries.
The country's head - Crown Prince Alois - has argued that Germany's investigation is illegal.
The Organisation for Economic Co-operation and Development lists Liechtenstein as one of only three states remaining on its blacklist of "unco-operative tax havens".