BERLIN (Nov 5): The world's biggest companies disclose little or no financial details about their operations abroad, according to a report by Transparency International which singled out Chinese firms but also U.S. tech giants like Amazon and Google.

The Berlin-based anti-corruption watchdog named Bank of China, Honda Motor, Bank of Communications , Agricultural Bank of China and Russia's Sberbank as the five least transparent companies.

Bank of China could not be reached for comment on the report, while the other four firms declined to comment.

"We need more transparency from multinational companies, whose power in the world economy closely rivals the biggest countries," said Transparency International chairman Jose Ugaz.

"By not responding to people's demands for greater transparency and accountability, companies risk harming their brand and losing customers," he warned.

Around three quarters of the 124 firms assessed do not disclose the taxes they pay in foreign countries and nearly half publish no information on revenues abroad, the report said.

The companies were ranked based on their reporting of their measures to prevent corruption, information about subsidiaries and holdings and information about financial operations abroad.

By these criteria, European companies performed best with Italy's Eni, Britain's Vodafone and Norway's Statoil at the top.

"We are committed to providing as much information and insight as possible to support informed public debate on issues such as corporate governance, anti-corruption programmes and corporate taxation," said a spokesman for Vodafone.

While there were six Chinese companies among the 10 worst performers, U.S. firms also figured.

Warren Buffet's conglomerate Berkshire Hathaway was ranked the sixth least transparent multinational and the report said major tech companies also performed relatively poorly.

"Surprisingly, the sector that makes greater transparency possible is one of the least transparent," said Transparency, adding that Amazon, Apple, Google and IBM all scored less than three out of 10.

The U.S. firms cited were either unavailable when asked for a reaction by Reuters or declined to comment. Some firms did not want to respond until they had seen the complete report.

A spokesperson for Japan's Nippon Telegraph and Telephone Corp, which was also among the bottom 10, said: "We want to continue to work towards appropriate disclosures."

Transparency said the world's biggest oil, gas and mining companies were not yet ready for transparency rules that would enter into force across the European Union from July 2015.

These regulations require extractive companies to report payments such as taxes to governments on a country-by-country and project-by-project basis. In the United States, similar measures are planned, but implementation has been delayed.