By Laurent Prieur

NOUAKCHOTT (Reuters) - A Mauritanian court on Tuesday sentenced one man to death and jailed two others for the 2009 murder of an American aid worker, a killing claimed by Islamist fighters operating in Africa's Sahara zones.

The trio -- alleged members of Al Qaeda's North African wing -- were on trial for the slaying of Christopher Leggett, who worked for a charity and was head of a language school when he was killed in the capital.

Mauritania is one of a string of West African nations bordering the Sahara desert that has been increasingly targeted by Islamists.

The court handed Mohamed Abdallahi Ould Ahmednah the death penalty for murdering the 39-year-old American. Sidi Mohamed Ould Bezeid was jailed for 12 years and Mahmoud Ould Khouna for three years, both for complicity.

All three were also convicted of belonging to a terrorist organization and having "attacked Mauritania."

Al Qaeda's branch in North Africa (AQIM) had claimed responsibility for the killing, saying Leggett was shot because he was trying to convert Muslims to Christianity.

Mauritania is among several countries in the Sahara region where al Qaeda-linked fighters have raised their profiles with a series of attacks and kidnappings.

AQIM grew out of the militant Salafist movement in Algeria and has moved south where it is taking advantage of the vast and lawless desert sahel regions of Mauritania, Mali and Niger.

(Writing by David Lewis; editing by Matthew Jones)