A Ukrainian accused of helping perpetrate the largest data theft ever was sentenced in Turkey last week to 30 years in prison after being convicted of breaking into Turkish consumers' bank accounts electronically.

Maksym Yastremskiy, 27, plans to appeal his sentence, his attorney said. Turkish prosecutors had requested a sentence of 24 to 70 years, according to a report published Friday in The Boston Globe.

Observers said the 30-year sentence was one of the longest ever handed down for a computer crime and probably would not be permitted in the United States unless it had led to other, more serious crimes, such as murder.

Last year U.S. prosecutors charged Mr. Yastremskiy and 10 other people with having played a key role in the theft and sale of about 29 million MasterCard Inc. card accounts and 65 million Visa Inc. accounts, from TJX Cos. Inc. and several other retailers.

Mr. Yastremskiy reportedly earned more than $11 million from the theft, according to U.S. prosecutors, and he has also been charged in another financial fraud case involving a Texas restaurant chain.

Another defense attorney said earlier that Turkish law would prohibit Mr. Yastremskiy from being extradited to the United States, as the Justice Department has requested, until he completes his sentence there.