United States tax authorities have found far fewer than 52,000 accounts with Swiss bank UBS suspected of being used to avoid taxes, the US government says.

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

US authorities are suing UBS in a civil case to try to obtain details about the 52,000 accounts.

"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 a court filing.

"There is no dispute that the vast bulk of information sought in this summons is not in the IRS's possession."

The IRS has declined to name how many US account names it has discovered.

UBS has argued repeatedly that it would violate Swiss banking secrecy laws if it complied with the US request to disclose the account details.

A hearing in the civil case is set for July 13, although an out-of-court settlement has not been ruled out.

UBS in February settled a US criminal investigation by paying $780 million (SFr849 million) and acknowledged that some of its employees helped US clients hide money.

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