The Internal Revenue Service has stepped up scrutiny of offshore accounts and foreign income, an enforcement campaign that could sweep up tens of thousands of taxpayers.

The push to recover some of the billions of dollars lost each year to offshore tax evasion goes beyond the government's high-profile effort to force Swiss bank UBS /quotes/comstock/13*!ubs/quotes/nls/ubs (UBS 13.46, -0.23, -1.68%) AG to release the names of 52,000 American account holders in order to nab tax evaders.

The IRS is using a once-obscure tax form called the Foreign Bank Account Report, or FBAR, to force taxpayers to provide information on income they earn or bank accounts they hold overseas. It is threatening tough action against taxpayers who don't file the form and has greatly broadened those subject to filings beyond direct owners of offshore accounts. The requirement applies to U.S. citizens and residents who have offshore accounts totaling $10,000 at any point during the year.