(Recasts with UBS confirmation, adds background)

By Lisa Jucca

ZURICH, Jan 9 (Reuters) - Swiss wealth manager UBS AG (UBSN.VX: Quote, Profile, Research) (UBS.N: Quote, Profile, Research) is closing all the offshore accounts of its U.S. clients, the bank said on Friday, as it comes under pressure from U.S. tax authorities.

The Swiss bank decided in July last year to stop offering offshore accounts to U.S. citizens after it was targeted by a U.S. tax investigation which challenges Switzerland's famous banking secrecy laws.

U.S. prosecutors have alleged UBS helped clients hide $18 billion of untaxed American money in undeclared accounts. This amounts to around $300 million of annual unpaid taxes, the newspaper said.

UBS spokesman Serge Steiner said the decision to close offshore accounts for U.S. domiciled clients was taken in November 2007. The bank started last year to close cash accounts of U.S. clients holding less than 50,000 Swiss francs ($45,660), he added.

"This is an ongoing process. It started last year and accelerated since last summer," Steiner said, confirming a New York Times report. As part of the investigation, U.S. authorities indicted UBS's wealth management chief last year.

UBS, which U.S. authorities say helped wealthy Americans hide cash in offshore bank accounts, will shut about 19,000 offshore accounts, the paper said, quoting unnamed U.S. clients.