Opening a new front in the government’s battle against Internet gambling, federal prosecutors have asked four American banks to freeze tens of millions of dollars in payments owed to people who play poker online.

The frozen payments of at least $33 million are owed to 27,000 players at four offshore poker sites, including PokerStars.com and FullTiltPoker.com, according to John Pappas, the executive director of the Poker Players Alliance, an advocacy group for online poker players.

Mr. Pappas said prosecutors in the last two weeks have asked Citibank , Wells Fargo and two smaller banks to freeze funds in accounts belonging to two companies, Allied Systems and Account Services, that process payouts on behalf of the poker sites.

The government action came to light over the weekend after checks issued to poker players by the two companies began bouncing, Mr. Pappas said. He said that the online casinos had assured him that they planned to pay players what they were owed.

Yusill Scribner, a spokeswoman for the office of the United States attorney for the Southern District of New York, which is bringing the legal action, declined to comment.

Stephen Cohen, a spokesman at Citibank, a unit of Citigroup, confirmed that the bank had received a request from prosecutors. He said that as a matter of policy Citibank complies with such requests.