With other African states that are members of ECOWAS considering military operations, several states connected to Niger have rushed to pledge support, many of whom have had coups recently.
The borders that have reopened are mainly in remote desert areas. Niger's key entryways for trade and commerce remain closed due to sanctions imposed by the regional bloc.
"Any military intervention against Niger would be considered as a declaration of war against Burkina Faso and Mali," a statement reads.
For a decade, offensive efforts have failed to stop an Islamist insurgency that has killed thousands and displaced millions.
The mass kidnapping is a first in the insurgency that spread to Burkina Faso from neighboring Mali in 2015 despite costly international military efforts to contain it.
The killings happened against the background of an Islamist insurgency spreading from Burkina Faso's neighbouring Mali.