Rich Chinese, watch out. The growing reach of U.S. tax authorities could get you in trouble at home.

A new U.S. tax rule known as Fatca, the Foreign Account Tax Compliance Act, is going into effect. Beginning this past Monday, more than 77,000 banks and other financial firms around the world started telling U.S. authorities about accounts owned by U.S. citizens and green-card holders.

Days before the rules hit, China rushed to take a first step to join in by signing an initial agreement with the U.S. Treasury.

Fatca has been criticized because it forces foreign banks, brokers and insurers to disclose information about U.S. customers. Foreign firms need to provide information about U.S. customer accounts totaling as little as $50,000.

But if they don't, the firms and their account holders could be docked 30% of their payments, such as interest and dividends, from U.S. sources. For example, Chinese investors who buy U.S. Treasuries through Bank of China could lose 30% of their interest payments.