US President Donald Trump asked a federal judge to allow him to review documents that FBI agents seized from the office of his long-time lawyer before criminal investigators have a chance to see the material.

The request underscores the high stakes in an ongoing legal fight in federal court in New York, where Michael Cohen, Trump's lawyer, is also fighting to get a chance to review material seized as part of a criminal investigation of his business dealings.

Trump's request, in the form of a letter from other lawyers representing him, could further complicate a hearing set for tomorrow.

During that session, lawyers for Cohen are expected to tell the judge overseeing the case how many legal clients he has and how many seized documents he thinks might be covered by lawyer-client privilege.

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Cohen is set to attend the hearing. Also expected to be on hand is adult-film star Stormy Daniels, whom Cohen secretly paid US$130,000 in 2016 to keep quiet the details of an alleged sexual liaison she had with Trump.

Prosecutors indicated in court filings last week that Cohen has been under criminal investigation for months by the US Attorney in Manhattan and that a grand jury has been hearing evidence in the case.