Signing tax information exchange agreements (TIEAs) is a step in the right direction - but offshore financial centres have to make sure they work in practice, said one of the US Congress's top tax investigators.

Bob Roach, counsel and chief investigator for the US Senate's Permanent Sub-committee on Investigations, which is chaired by Senator Carl Levin, said there were cross-border gaps in global regulation being exploited by tax cheats and their advisors.

And in a tough-talking demonstration of he US administration's hardening line on tax dodging, Mr. Roach said governments would "take action unilaterally" if it were necessary.

Offshore financial centres, including Bermuda, have signed a series of TIEAs in recent months as they try to meet the international tax transparency standard of at least 12 agreements. Bermuda and the US agreed a TIEA more than 20 years ago.

"The issue is not whether an agreement is in place, it's whether the agreement is being implemented fully or whether it's extensive enough to allow for effective exchange of information," said Mr. Roach.

Speaking in a panel discussion at the OffshoreAlert Financial Due Diligence annual conference in Miami, Mr. Roach added that trust advisors, lawyers and accountancy firms, as well as financial institutions, were helping individuals and corporations to cheat.

"Accountancy and law firms are helping to exploit gaps in regulation," Mr. Roach said to a conference audience including many from both professions. "They are helping to design financial transactions that become more and more opaque and complex by the day - transactions that have no benefits other than evading taxes."

Professionals who helped with such arrangements were "taking their cut and getting away scot-free", he added.

Some financial institutions were aiding and abetting US citizens to break their own country's laws, Mr. Roach said. In Liechtenstein, some bankers had been instructed to use pay phones or cell phones with Austrian numbers to leave as little evidence as possible of their dealings with clients.

He also took aim at Swiss bank UBS, which the US believes has helped many US clients to evade taxes.

"UBS is a flagship institution of Switzerland," Mr. Roach said. "It sent an armada of private bankers to the US to assist US citizens in evading tax and breaking securities laws. They directed clients on how to form offshore corporations and shirk US laws."

Liechtenstein had "jumped into action", when Germany started to apply pressure, in cooperating with larger countries' tax authorities. But Switzerland had been less helpful when the US came looking for information on 46,000 Americans who were UBS clients. In what Mr. Roach described as "a cynical move", UBS eventually offered information on just 12 of them.

The US and Switzerland have a tax treaty that has been in place for more than half a century. The terms are being renegotiated.

"The problem is global transparency," Mr. Roach said. "And until jurisdictions can exchange information without playing games with each other, then we will continue to struggle."

He also hit back at a claim of US hypocrisy from Cayman Islands Financial Services Association chairman Eduardo D'Angelo Silva, who argued that the biggest tax havens in the world were the US states of Delaware, Nevada and Wyoming, and also the UK.

Mr. Roach said he was just as concerned about the lack of transparency within those US states as elsewhere and said Sen. Levin supported proposed legislation to correct it.

"I think the problem is that our system is not in balance," Mr. Roach said. "The globalised system allows transactions to cross borders in seconds.

"Sometimes it's difficult for governments to regulate that international system and those gaps are being exploited by parties who aim to evade paying taxes on their income.

"The pathways being used to evade taxation are the same pathways being used for money laundering and the financing of terrorism."

The US believes it is losing around $100 billion in taxes on undeclared income, from both corporations shifting profits offshore and from high net worth individuals.

Last year, Sen. Levin tabled the Stop Tax Haven Abuse Act, which names Bermuda on its list of "secretive" jurisdictions, a bill co-sponsored by Barack Obama before he left the US Senate on being elected US President.

Sen. Levin tabled a beefed-up version of the same legislation earlier this year.