A German man accused of membership in the Somalia-based al-Shabab Islamic extremist group has been arrested at Frankfurt airport.

Federal prosecutors say the 28-year-old, identified only as Abshir Ahmed A. in line with German privacy rules, was arrested on Monday as he returned to Germany from Somalia. He is accused of membership in a foreign terrorist organization and preparing an act of violence.

Prosecutors said in a statement Tuesday that the man is believed to have joined al-Shabab in Somalia in 2012, undergoing weapons training and then being deployed at a defensive position for the group. They acted on an arrest warrant that was issued in March 2015.

The militants are waging an insurgency against Somalia's weak western-backed government.