Oklahoma City - A former missionary convicted of sexually abusing children at an orphanage in Kenya was sentenced on Monday to 40 years in a US prison.Matthew Lane Durham, 21, had faced up to 30 years on each of four counts of engaging in illicit sexual conduct in foreign places. He was ordered to pay restitution of $15 863.Durham asked the court for mercy. "All I wanted was to follow God's plan for me," he told the judge.Defence attorney Stephen Jones has said Durham plans to appeal. He has described Durham, who was 19 when he was charged, as "an emotionally vulnerable teenager" who was struggling with sexual identity while being a devout Christian.Prosecutors alleged Durham targeted orphans while volunteering at the Upendo Children's Home in Nairobi between April and June 2014. Durham had served as a volunteer since 2012 at the orphanage, which specialises in caring for neglected children.Orphanage officials and five of the children travelled from Kenya to testify at the trial.Prosecutors asked that Durham be placed under supervision for the rest of his life in the event he is ever released from prison."He raped or sexually molested by force or threat four children ranging in ages from 5 years to 14 years - some multiple times - in a span of just 33 days," prosecutors wrote.Prosecutors also said Durham's actions have had a chilling effect on foreign volunteers in Kenya and elsewhere "who must now live under the cloud of suspicion, distrust and apprehension when they volunteer their time, talent and resources for the betterment of children in East Africa and beyond."Evidence produced by prosecutors included handwritten, signed confessions that Durham gave orphanage officials after he was accused of inappropriate behaviour.Jones has argued that the statements were coerced by orphanage officials who isolated Durham, took his passport and created the allegations to obtain $17 000 from the US government for security cameras.