Word of the dispute emerged late Tuesday when Citi issued a brief statement saying it had been hit with an arbitration claim from the Abu Dhabi Investment Authority (ADIA) alleging "fraudulent misrepresentations" in connection with the two-year-old agreement.

ADIA, one of the world's biggest government wealth funds, declined to provide a copy of the claim or discuss the dispute in detail.

"It is the policy of ADIA to pursue its legal rights fully," an ADIA spokesman said. "ADIA declines to comment further due to binding confidentiality obligations, which ADIA intends to respect."

Citigroup earlier promised to "vigorously" fight the claims, which it said had no merit. It said ADIA was seeking to terminate the deal, made at a time the New York-based bank needed capital to offset big losses from mortgages and other investments, or receive more than US$4 billion in damages.

Details of the legal battle remain murky. A Dubai-based spokesman for Citigroup had no additional comment Wednesday.

In exchange for its November 2007 investment, ADIA received equity units that paid a high annual dividend.

The units were to be converted into Citigroup common shares at a price of up to US$37.24 each, starting next March and continuing through September 2011, making the fund one of Citi's largest shareholders with a 4.9 per cent stake.

"The structure in place seems to limit ADIA's ability to manoeuvre ... and exit when they want, either at a profit or a loss," said Rachel Ziemba, an analyst at Roubini Global Economics who monitors Gulf economies.

She speculated that the fund's legal action might be a negotiating tactic designed to increase its leverage. "It could be an attempt to get a better deal," she said.

At the time, the fund's managing director said the investment reflected its confidence in "Citi's potential to build shareholder value."

Hard hit by credit crisis

Since then, Citi has proved to be among the lenders hardest hit by the credit crisis and rising loan defaults. It received one of the biggest US government bailout packages - US$45 billion in aid and protection against losses on more than US$300 billion in assets.

Citi shares have plunged 89 per cent, from the US$33 range at the time the ADIA deal was made, to less than US$4 now.

At US$37.24 per share, the conversion price would amount to more than 10 times Citi's closing stock price on Tuesday of US$3.56.

"They probably feel they didn't quite get what they signed up for," Ziemba said. "They might not have priced in how vulnerable the US financial system was at the time. They weren't the only ones."

ADIA is the largest of several investment funds Abu Dhabi uses to invest its oil wealth. The size of its holdings has not been made public, but it is believed by some to be the world's largest sovereign wealth fund.

Estimates of its size have ranged from less than US$400 billion to US$875 billion and up.

Federation capital

Abu Dhabi is one of seven semi-autonomous sheikdoms that make up the United Arab Emirates, among OPEC's top five oil producers. It serves as the federation's capital, with control over the presidency and nearly all the country's oil reserves.

On Monday, Abu Dhabi's government agreed to pump an additional US$10 billion in bailout funds into its struggling neighbour Dubai to keep one of that sheikdom's star companies from defaulting on a loan.

That same day, Citi announced a deal to repay US$20 billion in bailout aid. The US government also will sell its 34 per cent stake in Citi, which it had swapped for US$25 billion in bailout loans.

Last week, Kuwait's sovereign wealth fund booked a profit of US$1.1 billion by selling the stake it took in Citigroup after ADIA's investment less than two years ago.

The Government of Singapore Investment Corp, another government fund that came to Citi's aid during the credit crisis, realised a US$1.6 billion profit in September when it cut its stake in the bank to below five per cent.

- AP