WASHINGTON, July 25 (Reuters) - The U.S. Senate's Finance Committee on Tuesday revealed an ongoing probe into private equity billionaire Leon Black's financial ties with disgraced late financier Jeffrey Epstein, and said the investigation "uncovered serious tax issues."

"The investigation has uncovered serious tax issues and other concerns with trusts and structures Black executed to avoid over $1 billion in future gift and estate taxes," the panel said in a statement on Tuesday, adding its probe began in June 2022.

The Senate panel said that a reported $158 million of payments in several installments from 2012 to 2017 by Black to Epstein for financial advice seemed "inexplicably large," given that Epstein was "neither a licensed tax attorney nor a certified public accountant."

The panel also alleged Black "has refused to answer questions or provide any documents that could demonstrate how Epstein's compensation for tax and estate planning services was determined or justified."

A spokesperson for Black said the billionaire had "cooperated extensively" with the panel's probe and provided detailed information.

"The transactions referenced in the Committee's letter were lawful in all respects, were conceived of, vetted and implemented by reputable law firms and tax and other advisors, and Mr. Black has fully paid all taxes owed to the government," Black's spokesperson said in an emailed statement.

Black had previously acknowledged he paid Epstein for "legitimate financial advisory services." Last week, the New York Times reported that Black paid $62.5 million to the U.S. Virgin Islands to avoid any legal claims tied to an Epstein sex-trafficking investigation.

Black is worth $10.1 billion, according to Forbes magazine. He left Apollo Global Management, the private equity firm he co-founded, in 2021.

Epstein, a registered sex offender who once counted former presidents Donald Trump and Bill Clinton as friends, killed himself in 2019 while awaiting trial on sex-trafficking charges. (Reporting by Kanishka Singh in Washington; editing by Jonathan Oatis)