More than 15 years after El-Sayyid A. Nosair was convicted of seditious conspiracy and of murdering Rabbi Meir Kahane, a dispute has erupted over a claim by Mr. Nosair that prosecutors improperly blocked his trial lawyers from gaining access to a witness who he says would have supported his claim of innocence, court papers show.

A federal magistrate judge recently ordered a hearing on Mr. Nosair’s claim of prosecutorial misconduct, leading to a sharp response from prosecutors, who say Mr. Nosair’s contention is “absurd” and deny suppressing the witness.

The government also argues that, given what is now known about the witness, a former United States Army sergeant who became a Qaeda operative and a close aide to Osama bin Laden, the only way he could have provided exculpatory testimony for Mr. Nosair would have been to lie on the stand.

“A defendant has no right to present perjured testimony,” a federal prosecutor, Michael D. Lockard, wrote to Judge Richard J. Holwell of the United States District Court in Manhattan, “and Nosair can hardly invoke the court’s aid to vacate his conviction based on such a claim.”

Mr. Lockard asked Judge Holwell to overturn the order for a hearing, which prosecutors described as “clearly erroneous and contrary to law.” Prosecutors have also asked that the judge dismiss Mr. Nosair’s petition, which seeks a new trial.

The hearing has been postponed pending Judge Holwell’s review, but the dispute is a reminder of how intertwined the threads of Islamic terrorism have been in New York since the killing of Rabbi Kahane, the founder of the Jewish Defense League, and even before that.

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