HOUSTON, May 15 (Reuters) - The court-appointed attorney overseeing the operations and assets of Texas financier Allen Stanford's businesses asked a federal judge on Friday to approve $20 million in fees and expenses for firms that have done work for him, according to court records.

"The tasks and challenges presented by this large receivership are numerous, complex, difficult, and in some respects unprecedented, Ralph Janvey, the receiver, said in a 51-page filing with federal court in Dallas.

Stanford, two of his top aides and three of his companies are accused of an $8.5 billion fraud by the U.S. Securities and Exchange Commission. (Reporting by Anna Driver in Houston)