Shares in UBS opened lower on Monday as the Swiss bank faced more tense negotiations to settle a tax evasion case in the U.S. that challenges Switzerland's bank secrecy laws.

The Swiss government, currently in recess, will hold an extraordinary meeting later on Monday to discuss the issue, a government official said, without adding details.

Switzerland and the United States failed on Friday to agree on a settlement that would spare UBS a tax trial, and talks will continue until August 12, when a new status conference with U.S. Judge Alan Gold is scheduled.

Berne and Washington have already reached an agreement in principle on the major issues, and a settlement is expected to involve the disclosure to U.S. tax authorities of the names of some of the 52,000 U.S. clients of UBS holding secret Swiss accounts.

UBS shares were down 1.5 percent at 16.10 Swiss francs at 3:22 a.m. EDT, underperforming the DJ Stoxx European Banking index, which was down 0.4 percent <.SX7P>.

Swiss media have said U.S.-Swiss talks have stalled on legal details on how to allow the transfer of some client data to Washington.

It is not a problem that has to do with UBS, but rather with the legal procedure, Finance Minister Hans-Rudolf Merz told Swiss television on Sunday, without going into details.

According to Swiss newspaper NZZ am Sonntag, the United States wants guarantees that the so-called administrative assistance process, the legal framework under which bank client details can be transferred to the United States, will deliver the data certainly and quickly.

But Merz said on Sunday Switzerland was not prepared to introduce emergency measures to force the pace of the data-handling process.

The case against UBS is set for court on August 17, but if the parties choose to delay further, the next available date would be September 21.

Berne already stretched its legal system in February when it forced UBS to quickly hand over 250 client names, bypassing the clients' appeals, in order to settle criminal charges against the bank.

(Writing by Lisa Jucca, editing by Will Waterman)