By Joseph Ax

NEW YORK (Reuters) - An accused al Qaeda operative made terrorism "his life's work," including his role in a 2003 attack that killed two American servicemen in Afghanistan, federal prosecutors said on Monday at the start of his criminal trial.

Ibrahim Suleiman Adnan Adam Harun, known by the nom de guerre Spin Ghul, also plotted to bomb the U.S. Embassy in Nigeria before his capture 12 years ago, Assistant U.S. Attorney Matt Jacobs told a jury in Brooklyn.

"The defendant embraced terrorism at a young age and made it his life's work," he said.

Harun's defense lawyer, Susan Kelman, confirmed that Harun had pledged allegiance to al Qaeda but told jurors not to allow "fear and prejudice" to sway them into convicting him of every crime the government had alleged.

"Mr. Harun may not share your worldview, but is that enough to convict?" she said without offering many details of the defense she plans to mount.

Her task has been made difficult by Harun's refusal to participate in the case. He was not present in court on Monday and has not spoken to his court-appointed lawyers in two years.

Since his extradition from Italy in October 2012, the Saudi-born defendant has insisted he is a "warrior" who should face a military tribunal rather than criminal proceedings.

Before an appearance in May, Harun scuffled with U.S. marshals and tore off his clothes before disrupting the hearing by screaming from an adjacent holding cell.

U.S. District Judge Brian Cogan permitted Harun to watch by video from jail, though it is not clear whether he was doing so.

Harun, who claims Niger citizenship, was captured in Libya in 2005 and released in 2011 to a refugee ship headed for Italy before Italian authorities seized him and notified U.S. federal agents.

During a three-day hearing in an Italian court, Harun recounted his experiences in great detail, Jacobs said.

Harun admitted he joined al Qaeda in Afghanistan shortly before the Sept. 11, 2001, attacks on U.S. soil and underwent extensive arms training. Soon after U.S.-led forces invaded the country, Harun and other fighters fled to the mountainous region along the Pakistani border.

The attack on U.S. troops killed Army Private 1st Class Jerod Dennis, 19, and Air Force Airman 1st Class Raymond Losano, 24. Some relatives of Dennis attended court on Monday but declined to speak with reporters.

Eventually, Harun traveled to Nigeria, where he plotted to bomb the U.S. embassy, Jacobs said.

(Additional reporting by Nate Raymond; Editing by Peter Cooney and Cynthia Osterman)