The announcement was made through the Federation’s official Twitter account.
“Coach Oliseh has returned to Belgium to continue treatment after being advised by Eagles medics.
“He became dizzy and weak after lunch on Thursday and he travels abroad tonight through the Port Harcourt Airport,” the NFF said.
Oliseh is expected to meet with his personal doctor in Belgium for what is believed to be a worsening case of flu.
The technician will not be on the bench when Eagles take on Burkina Faso on Sunday in the second match of the qualifier for the 2016 Championship for African Nations (CHAN).
Initial reports indicate that he took ill soon after Nigeria defeated Cameroon 3-0 in a friendly match played in Brussels on October 11.
In his absence, Salisu Yusuf will take charge of the team in Ouagadougou on Sunday.
The Eagles B are travelling to the Burkina capital with a 2-0 advantage from the first leg on Saturday, October 17.