NEW YORK (: A defence lawyer for the son-in-law of Osama bin Laden convicted on terrorism-related charges in New York federal court said on Thursday that his client should serve no more than 15 years in prison.

Kuwaiti-born Suleiman Abu Ghaith, 48, faces up to life behind bars after a jury convicted him in March of conspiring to kill Americans, conspiring to provide material support for terrorists, and providing such support.

The trial, which featured testimony from Abu Ghaith himself, offered an unusual glimpse into bin Laden's actions in the days following al Qaeda's attacks in the United States on Sept. 11, 2001, that killed nearly 3,000 people.

Prosecutors accused Abu Ghaith of serving as an al Qaeda mouthpiece, recording inflammatory videos that the group used in recruiting.

Abu Ghaith's lawyer, Stanley Cohen, argued in a court filing that his client was convicted solely on the basis of speech.

"The defendant faces the harshest of penalties for talk - and only talk - which is at times zealous, pious and devout; at other times intemperate; at still others, offensive to core values of humanity," Cohen wrote. "In this sense, he was not unlike an outrageous daytime 'shock-radio' host, or a World War II radio propagandist for a losing ideology."

Prosecutors from the office of Manhattan US Attorney Preet Bharara also said at trial that Abu Ghaith knew in advance of an attempt to detonate a shoe bomb aboard a jetliner by Briton Richard Reid in December 2001, citing a video in October 2001 in which Abu Ghaith warned that the "storm of airplanes will not stop."

Defence lawyers argued that the case was based upon association, rather than actual evidence that Abu Ghaith was aware of any plots.

Thursday's filing also asserted that Abu Ghaith's speeches did not help recruit anyone and that he knew, even as he made them, that al Qaeda was likely doomed.

"His speeches ... are little more than an epic act of whistling past the graveyard," Cohen said.

The government has not yet submitted its sentencing recommendation. Abu Ghaith is set to be sentenced on Sept. 23.

Cohen himself pleaded guilty in May to tax-related charges in a separate case. He is expected to be sentenced to an 18-month in prison in early September.