By Nate Raymond

NEW YORK (Reuters) - U.S. prosecutors on Monday dropped an appeal seeking to lengthen the prison sentences of five former employees of Bernard Madoff convicted for helping the imprisoned fraudster carry out his multibillion dollar Ponzi scheme.

The five employees were sentenced last year to prison terms of 2-1/2 to 10 years, less than half the amount of time sought by federal prosecutors in Manhattan.

Their decision to drop a challenge of those sentences was disclosed in a court filing with 2nd U.S. Circuit Court of Appeals in New York.

Prosecutors gave no explanation for their decision. A spokesman for Manhattan U.S. Attorney Preet Bharara declined comment.

The five employees were among 15 people who pleaded guilty or were convicted at trial in connected to Madoff and his firm, Bernard L. Madoff Investment Securities LLC.

Madoff is serving a 150-year prison term after pleading guilty in 2009 to running a scheme that cost investors an estimated $17 billion or more in principal.

U.S. District Judge Laura Taylor Swain sentenced the five employees in December 2014, following their convictions that March.

Former back office director Daniel Bonventre received 10 years in prison; portfolio managers Joann Crupi and Annette Bongiorno received six years; and computer programmers Jerome O'Hara and George Perez received 2-1/2 years each.

Prosecutors had requested more than 20 years for Bonventre and Bongiorno, more than 14 for Crupi and more than eight for O'Hara and Perez.

The employees are separately appealing. Larry Krantz, a lawyer for Perez, said he was pleased prosecutors dropped their appeal of his client's sentence.

"We have always believed that the appeal lacked any merit," he said.

The case is U.S. v. O'Hara, 2nd U.S. Circuit Court of Appeals, No. 15-64.

(Reporting by Nate Raymond; Editing by Lisa Shumaker)