In a move that could further strain Washington's already fraught relations with Afghan President Hamid Karzai, federal prosecutors are considering an indictment against his brother for tax evasion, according to a U.S. official and a source familiar with the investigation, The Washington Post reports.

The U.S. attorney's office in Manhattan, assisted by Internal Revenue Service agents and others, has been looking into the finances of President Karzai's older brother, Mahmoud, the sources said.

In a telephone interview from Kabul, Mahmoud Karzai - a U.S. citizen who used to live in Maryland - said he had not been contacted by U.S. authorities and added: "I know myself. I'm very clean."

A source familiar with the investigation said Mahmoud Karzai is expected to be indicted by a federal grand jury in U.S. District Court in Manhattan "The $64,000 question is when," said the source, who spoke on the condition of anonymity because no indictment has been made public.

Justice Department officials and federal prosecutors in Manhattan declined to comment. Given the diplomatic sensitivities, high-level officials at the Justice Department and possibly elsewhere in the government are likely to be involved in the final decision about whether to bring charges.

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