* Senator Wyden wants public hearing on corporate tax
* Cayman Islands, Bermuda common sites of shell companies
* U.S. multinationals avoid $90 bln/year in taxes - report
* Business-aligned research group says report "misleading"
(Adds details on hearing, background on inversions)
WASHINGTON, June 5 (Reuters) - Most Fortune 500 corporations have offshore tax haven units that they use to avoid paying U.S. taxes through "accounting tricks," two left-leaning tax activist groups said on Thursday.
U.S. multinationals each year avoid paying about $90 billion in federal income tax, the groups said in a report that comes amid increased attention to corporate tax avoidance worldwide.
"Many large, U.S.-based multinational corporations avoid paying U.S. taxes by using accounting tricks to make profits made in America appear to be generated in offshore tax havens - countries with minimal or no taxes," said Citizens for Tax Justice (CTJ) and U.S. Public Interest Research Group (PIRG).
About 72 percent of Fortune 500 companies had subsidiaries in low-tax jurisdictions in 2013, with most of these units located in the Cayman Islands or Bermuda, said the report.
"These subsidiaries are often shell companies with few, if any employees, and which engage in little to no real business activity," it said.
Big corporations regularly defend their tax planning practices as legal and in the best interest of shareholders, who want companies to pay as little tax as possible.
But fiscal constraints facing many governments, as well as the increased aggressiveness recently of corporate tax-avoidance strategies, have fueled a political backlash.
WYDEN TO EYE 'INVERSIONS'
Democratic Senator Ron Wyden, the U.S. Senate's top tax law writer, said he will hold a hearing on corporate taxation in July, with a specific strategy likely to be a major focus.
Deals known as "inversions," in which a U.S. company moves its tax base to another, lower-tax country to cut its tax bills, are sure to come up at the hearing, Wyden told reporters.
"The pace of these inversions has accelerated," said the Senate Finance Committee chairman. "You really can't have a debate about the international aspects of the U.S. tax code without looking at inversions,"
Two major U.S. companies - drugmaker Pfizer Inc and advertising firm Omnicom Group Inc - recently tried to do inversions, but the deals fell apart for non-tax reasons.
The CTJ/PIRG report was criticized by the Tax Foundation, a business-aligned tax activist group, as "misleading."
"There's no doubt that U.S. corporations use a variety of legal methods to reduce their corporate tax bill on their overseas operations," the foundation said. "If we are going to reform our tax code, it is best to be informed by the best information available. This study doesn't cut it."
Earlier this week, the Paris-based Organisation for Economic Co-operation and Development held a conference in Washington on its effort to rein in corporate tax avoidance, launched last year at the request of the G20 group of leading world economies.
The OECD project has a long way to go, with completion not expected until next year, and it will not have the force of law, but lawyers and accountants agree that changes are coming.
(Editing by Jonathan Oatis)