U.S. tax authorities have discovered far fewer than the 52,000 accounts with Swiss bank UBS AG suspected of being used by Americans to avoid taxes, the U.S. government said in a court filing on Monday.

The U.S. Justice Department was responding to UBS, which asked a federal judge in Miami last week to order the Internal Revenue Service to reveal how many undeclared accounts of U.S. clients it has discovered at the bank.

The federal government is suing UBS in a civil case to obtain details on the 52,000 accounts. In February, UBS settled a U.S. criminal probe for $780 million and acknowledged that some of its employees helped U.S. clients hide money.

The number of undisclosed accounts that the IRS has discovered through 'alternative means' falls far short of the 52,000 accounts that UBS has identified, the Justice Department said in its latest court filing. There is no dispute that the vast bulk of information sought in this summons is not in the IRS's possession.

UBS has argued that the IRS has alternative ways to get the information it seeks. UBS says it would violate Swiss banking secrecy laws if it complied with the U.S. request.

The IRS has declined to name how many U.S. account names it has discovered. The agency has reported an increase in individuals coming forward as part of a voluntary disclosure program that generally lets them avoid criminal prosecution in exchange for declaring their accounts and paying monetary penalties.

A hearing in the civil case is set for July 13 in Florida.

Last month, the Justice Department denied it would drop the case, responding to a newspaper report citing anonymous sources that it was mulling that option.

The case is: United States v. UBS, U.S. District Court, Southern District of Florida, No. 09-20423.

(Reporting by Kim Dixon; Editing by Tim Dobbyn)