An American missionary has been sentenced to 40 years in prison for sexually assaulting three girls and one boy while volunteering at a children's home in Kenya.

Matthew Lane Durham, 21, 'raped three girls – ages five, nine and 15 – at least eight times', prosecutors told the court, in 'a span of just 33 days'.

During that same time period, they said, he sexually molested a 12-year-old boy twice.

A jury found Durham guilty on seven counts and judge David L Russell sentenced him to four decades behind bars, according to CNN.

Durham's attorney told the cable channel this week he plans to appeal the decision.

The region in east Africa where Durham served has been badly shaken by the incident, prosecutors wrote in a sentencing memorandum in February.

They wrote Durham's actions 'have had a chilling effect on the lives of dozens of foreign volunteers in Kenya and elsewhere who must now live under the cloud of suspicion...there is a real perception among Upendo's local Kenyan community that more paedophiles lurk among the volunteers, especially the male volunteers'.

It comes after he volunteered to care for neglected children at Upendo Children's Home in Nairobi, according to the criminal complaint.

It said Durham asked to stay in an 'overflow bunk' in the school rather than an off-site facilities with sponsor families to be in a 'better position to assist the children'.

A school caretaker noticed odd behaviour between Durham and the children, including 'lingering embraces' and 'lying beside some of the children on their beds' at night, the complaint says.

Leaders at the school confronted Durham and then held his passport, the complaint says. They returned the passport to him days later, and he flew back to Oklahoma. At the time of his arrest, Durham's attorney told CNN Durham was coerced into a confession to get his passport back.