Citibank, the largest credit card issuer in the U.S., has agreed to block all online gambling transactions that use its credit cards.

The agreement, announced by the bank and New York Attorney General Eliot Spitzer, applies to all Internet gambling transactions - not just those in New York - and goes into effect in 60 days.

"Americans now waste $4 billion a year on this pernicious form of gambling," Spitzer said, as quoted in an Associated Press report. "With this agreement, we will cut off an enormous line of credit that was a jackpot for illegal offshore casinos."

Citibank controls about 12 percent of the nation's credit card market. Bank of America, MBNA and Chase Manhattan Bank have also begun blocking the gambling transactions.

"Citibank agreed to take these steps to help alleviate concerns raised by the attorney general about the impact that gambling on credit may have on New York residents," said Citibank spokeswoman Maria Mendler.

Credit cards are only one of several methods players use to buy credits at online casinos.