The IRS, in an announcement planned for Tuesday, will give foreign-account holders until April 15 to cooperate, according to congressional sources briefed on the new initiative.

After that date, the IRS plans to pursue account holders and promoters aggressively, with tough statutes that carry prison sentences of up to 20 years.

The government estimates that 1 million U.S. citizens hold undisclosed credit cards from offshore banks. While the cards are not illegal, the law requires taxpayers to report ownership of foreign accounts.

Bank secrecy laws in 30 tax-haven countries have enabled people to hide large incomes, and evade U.S. income taxes, while living lavishly with their plastic.

The IRS began obtaining records in 2000 from Visa, MasterCard and American Express to target people for audits and prosecution for tax evasion. The IRS so far has about 1,000 active cases, including names of prominent and famous Americans, congressional sources said Monday.

Sen. Charles Grassley, R-Iowa, chairman of the Senate Finance Committee, "is pleased with the IRS' activity in this area and looks forward to tomorrow's announcement," said his spokeswoman, Jill Gerber. She would not discuss details of the proposal.

The investigation is among the largest ever to target the use of offshore bank accounts to hide income that should be subject to U.S. income taxes.

Some taxpayers even have sought bankruptcy protection from tax debts while continuing to draw on foreign accounts through credit cards issued by foreign banks.