Switzerland will start negotiations with the United States over its existing double-taxation treaty at the end of April, the Swiss ambassador to the United States told Swiss television late on Friday.

The first discussions already took place in Washington last week. We have agreed a date with the USA. The formal negotiations will start at the end of April -- hopefully in Switzerland, Urs Ziswiler said in an interview with Swiss news programme 10 vor 10.

Last month, President Hans-Rudolf Merz said Switzerland would approach the U.S. and Japan to review existing agreements as the country looks to offer more cooperation in the fight against tax evasion.

Earlier this week, the OECD placed Costa Rica, Malaysia, the Philippines and Uruguay on its blacklist of non-cooperative tax havens in a bid to crack down on tax evasion.

Switzerland was placed on a grey list of countries, which includes those that have agreed to improve transparency standards but have not yet signed the necessary international accords.

Discussions with the United States are key for Switzerland in light of a high-profile tax fraud investigation involving Switzerland's largest bank UBS that has forced Berne to hand over confidential bank client data to Washington.

Switzerland is also taking steps to defend the country's position in a civil case against UBS that seeks to force the bank to reveal the details of 52,000 bank clients.

Ziswiler said that concessions would have to be made regarding the disclosure of data of these UBS customers.

We cannot accept that parallel to these negotiations, there is a Damocles sword hanging over us in the form of the 52,000 names that a U.S. court is demanding from UBS. We cannot sign if progress is not made there, Ziswiler said.

(Reporting by Katie Reid)