NEW YORK (AFP) - A New York jury on Tuesday found Osama bin Laden's son-in-law and former Al-Qaeda spokesman Suleiman Abu Ghaith guilty of three counts of conspiracy to kill Americans and supporting terrorists.

The 48-year-old preacher from Kuwait, whose three-week trial has been the most high-profile Al-Qaeda case to reach a United States federal court, now faces life behind bars.

US prosecutors had alleged Abu Ghaith was an Al-Qaeda insider and Osama's right-hand man, and inspired a new generation of terrorists as the group's main messenger after the 9/11 attacks.

The outcome of the trial will be closely watched as pressure builds on the White House to close the Guantanamo Bay prison camp, and seen by some as proof that detainees can face civil justice.

Abu Ghaith denied the charges, but is most famous for appearing with Osama in a video on Sept 12, 2001 claiming the attacks on US targets that killed nearly 3,000 people the day before.

In follow-up videos, he threatened a "storm of airplanes", proof the government said that he was implicated in the December 2001 plot to blow up a transatlantic flight from Paris with a shoe bomb.

Abu Ghaith fled Afghanistan for Iran in 2002. He was arrested in Turkey in 2013 and sent to Jordan, where he was transferred to US custody.