WASHINGTON -- Wealthy Americans are hiding about $1.5 trillion in overseas tax havens in a "deceptive" partnership with top foreign banks such as UBS, resulting in $100 billion in lost US tax revenue, a congressional probe was told Thursday.

The hearing centered on a 115-page report following investigations into alleged abuses by UBS in Switzerland and the smaller LGT Bank in Liechtenstein amid a widening international investigation into tax scandals involving the two banks.

"The evidence we have been able to obtain breaks through some of the wall of secrecy to show that these two banks have employed banking practices that facilitate, and have resulted in, tax evasion by US clients," said Democratic Senator Carl Levin, who led the six-month US investigations.

The offshore tax evasion problem faced by the United States is of "staggering proportions," said Republican Senator Norm Coleman, also a leading member in the Senate investigations panel.

"These tax havens hold an estimated $1.5 trillion in American assets, resulting in lost taxes of roughly $100 billion," he said, describing some of the tax haven banking activities as "a cloak-and-dagger deception in a James Bond movie."

The OECD group of mostly industrialized economies estimates that about $5-7 trillion are held in tax havens or banking secrecy jurisdictions globally.

In response to inquiries by the Senate probe panel, UBS, the world's largest manager of private wealth, said 19,000 of the 20,000 accounts in Switzerland for US clients were "undeclared" in the range of nearly $18 billion.

US authorities have initiated enforcement action against 100 US taxpayers in connection with accounts in the secretive European principality Liechtenstein after a former LGT employee provided tax authorities around the world with data on about 1,400 people with bank accounts.

The ex-employee, who has gone into hiding, gave the US probe panel 12,000 pages of documents related to US clients as well as some, according to Levin, "explosive" evidence.

Former UBS banker Bradley Birkenfield had already pleaded guilty to conspiring to help US clients evade millions of dollars in taxes by hiding assets in Switzerland and Liechtenstein.

Another UBS official was detained as a "material witness."

The enforcement actions appear to represent the first time that the United States has criminally prosecuted a Swiss banker for helping a US taxpayer evade US taxes.

"The details of their tax evasion scheme are sordid enough," Levin said.

The Senate panel had also obtained a document showing that UBS provided its Swiss private bankers with training on how to detect surveillance by US customs agents and law enforcement officers while travelling to the United States, he said.

UBS had cooperated with the panel and one of its officials from Switzerland flew down to testify at the hearing.

LGT, which is owned by the Prince of Liechtenstein Foundation, denied the Senate's accusations and refused to appear at the hearing.

UBS told the panel it was "changing its ways," Levin said, citing a ban on travel by Swiss bankers to the United States and a request to US clients to disclose their accounts to the tax authorities.

Liechtenstein told the panel it was in negotiations with the United States to enter into a tax information exchange agreement with its European neighbors to expand tax cooperation in connection with an anti-fraud pact.

Levin also showed a chart summarizing tax haven bank secrecy "tricks" uncovered during the probe -- banks using code names for clients, telling staff to use pay phones instead of business phones so authorities cannot trace calls and giving staff encrypted computers when they travel so authorities cannot read any client information.

Other methods included funneling money through so-called transfer companies to cover the tracks of the funds and make audit difficult, and opening accounts in the names of foreign shell companies to hide the real owners.

"The documents and testimony that we are releasing today disclose a culture of secrecy and deception that we are determined to end, despite its being strongly entrenched," Levin said.