Indian billionaire Gautam Adani has been indicted by US prosecutors for his alleged role in a $US265 million ($A407 million) scheme to bribe Indian officials, plunging his conglomerate deep into crisis for the second time in two years.
Multiple counts of fraud - which US authorities say involved a firm that was listed in New York and affected American investors - were levelled against Adani, who is one of the world's richest people, and seven other defendants.
Shares and bonds of Adani firms tumbled on Thursday and Adani Green Energy, the company at the centre of the allegations, also cancelled a $US600 million bond sale.
Arrest warrants have been issued in the US for Adani and his nephew Sagar and prosecutors plan to hand those warrants to foreign law enforcement, court records show.
Adani Group said in a statement that the allegations were "baseless and denied", adding it would seek "all possible legal recourse".
US federal prosecutors said the defendants agreed to pay the bribes to Indian government officials to obtain power supply contracts expected to yield $US2 billion of profit over 20 years, and develop India's largest solar power plant project.
They also said the Adanis and another executive at Adani Green Energy, former CEO Vneet Jaain, raised more than $US3 billion in loans and bonds by hiding their corruption from lenders and investors.
The three were charged with securities fraud, securities fraud conspiracy and wire fraud conspiracy.
The Adanis were also charged in a parallel US Securities and Exchange Commission (SEC) civil case.
"Gautam and Sagar Adani were engaged in the bribery scheme during a September 2021 note offering by Adani Green that raised $US750 million, including approximately $US175 million from US investors," the SEC said in a statement, adding a company central to the scheme, Azure Power, used to trade on the New York Stock Exchange.
"The Adani Green offering materials included statements about its anti-corruption and anti-bribery efforts that were materially false or misleading in light of Gautam and Sagar Adani's conduct."
The charges follow much turmoil for the Adani Group in 2023 after short-seller Hindenburg Research issued a report that accused it of using offshore tax havens improperly, which the company has denied.
Indian authorities, including the Securities and Exchange Board of India, did not respond to requests for comment on the US charges.
Azure Power also did not respond to a request for comment.
Prime Minister Narendra Modi's administration has been accused by political opponents of favouritism toward Adani in government decisions.
Modi and Adani, who both are from the western state of Gujarat, have denied impropriety.
On Thursday, India's Congress party reiterated calls for a parliamentary investigation into alleged wrongdoing by the Adani Group.
The unsealed criminal charges by the US Attorney's Office for the Eastern District of New York showed some conspirators referred privately to Gautam Adani with the code names "Numero uno" and "the big man", while Sagar Adani allegedly used his cellphone to track specifics about the bribes.
Five other defendants were charged with conspiring to violate the Foreign Corrupt Practices Act, a US anti-bribery law, and four were charged with conspiring to obstruct justice.
None of the defendants is in custody, a spokesperson for US Attorney Breon Peace in Brooklyn said.
Gautam Adani is believed to be in India.
Gautam Adani is worth $US69.8 billion according to Forbes magazine, making him India's second-richest man after Mukesh Ambani.
He is one of the few billionaires formally accused in the United States of criminal wrongdoing.
