This frame grab shows Suleiman Abu Ghaith, a spokesman of al-Qaeda, claiming responsibility for the September 11 attacks in the United States in an undated videotape broadcast in 2002. File picture: Middle East Broadcasting Centre

New York - The United States on Monday said Suleiman Abu Ghaith, a son-in-law of Osama bin Laden, should spend the rest of his life in prison following his conviction on terrorism-related charges.

Abu Ghaith, 48, had been convicted in March by a Manhattan federal jury for conspiring to kill Americans, conspiring to provide material support for terrorists, and providing such support.

In a court filing, prosecutors portrayed Abu Ghaith as a charismatic mouthpiece for al-Qaeda after the September 11, 2001, attacks who recorded inflammatory videos to recruit new members, and said the defendant showed no remorse and lied at his trial.

Prosecutors also rejected arguments by defence lawyers, who seek a 15-year prison term, that Abu Ghaith did not knowingly plot to murder Americans, and that his speeches amounted to mere “whistling past the graveyard” because al-Qaeda was already reeling.

“The defendant played a central role in al-Qaeda's conspiracy to kill Americans, unapologetically celebrating the murder of nearly 3 000 people, proudly proclaiming to the world that this senseless tragedy was justified and America deserved to be attacked, boldly threatening more attacks of similar magnitude, and working to drive more fighters to join al-Qaeda's deadly mission,” prosecutors said.

“A sentence of life imprisonment is the only appropriate sentence in this case,” they added.

Stanley Cohen, a lawyer for Abu Ghaith, did not immediately respond to requests for comment.

The sentencing is scheduled for September 23 before US District Judge Lewis Kaplan in Manhattan.

Abu Ghaith's trial featured testimony from the defendant himself, and offered a glimpse into Bin Laden's activity shortly after the September 11 attacks.

The attackers flew two hijacked planes into the World Trade Center in New York and a third plane into the Pentagon outside Washington. A fourth hijacked plane crashed in a Pennsylvania field after passengers revolted.

US forces in Pakistan killed Bin Laden in May 2011. - Reuters