Yemeni employees of the U.S. government have been detained in Sanaa, where the compound that housed the American Embassy was breached by Iran-backed Houthi rebels who took control of many of war-torn Yemen’s cities in 2014, according to the State Department.

A spokesperson for the State Department said Thursday that a “majority” of the U.S. Embassy staff that were detained have been released and that Washington was engaging in “unceasing” diplomatic efforts to free the employees still in custody. It is not clear why the Yemeni employees were placed in custody, and State Department spokesman Ned Price said Tuesday that he was not able to say how many were detained.