NEW YORK—The government told a federal judge on Tuesday he doesn't have authority to release an ailing disbarred civil rights lawyer serving a 10-year prison sentence for letting an imprisoned blind Egyptian sheik communicate with his followers.

Prosecutors said in court papers that a request by Lynne Stewart's lawyers for her to be released so she can better fight terminal cancer should be rejected. They wrote that U.S. District Judge John G. Koeltl doesn't have the authority to reconsider or modify a term of imprisonment previously imposed unless the federal Bureau of Prisons requests it.

Defense lawyers last week asked the judge to free the 73-year-old Stewart after the director of the Bureau of Prisons rejected a compassionate-release request from Stewart in June, saying she had more than 18 months to live. Her lawyers, though, say her condition is "rapidly deteriorating" and she will soon succumb to breast cancer.

Prosecutors, in their papers, recounted Stewart's crime, saying she "materially assisted terrorists who were striving to kill people outside the United States and who only through fortuity failed in achieving their murderous objectives—although Stewart and her co-defendants' actions came perilously close to unleashing a violent terrorist attack."

At trial, jurors heard evidence that Stewart relayed a message from her client, Sheik Omar Abdel-Rahman, to the media despite special administrative measures taken to ensure he could not communicate with followers. The sheik is serving a life sentence for conspiracies to blow up New York landmarks and assassinate then-Egyptian President Hosni Mubarak. Stewart, who's been imprisoned since 2009, represented the sheik at his 1995 trial and afterward.

Stewart is being held at the Federal Medical Center, Carswell, in Fort Worth, Texas. Her cancer was first diagnosed in 2005 and was rediscovered last summer.

A hearing is scheduled for Thursday.