"From now on, we say, whether you're from Mali, Niger or Burkina, we have the same destiny. We're going in together," said Burkinabe premier Appolinaire Joachim Kyelem de Tambela
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.
Niger's junta denounced the comments as divisive and served only to perpetrate France's neo-colonial relationship.
Gabon's coup is only the latest in countries throughout Africa's Sahel region.
The deterioration in Niger-France relations echoes post-coup developments in Mali and Burkina Faso, which have booted out French forces and severed long-standing ties.
Perceived-Western backing of state actors could create future divisions in an already tense climate.
Niger is one of the least prosperous countries in the world, but has critical strategic value for several reasons.
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.
There have been at least seven other coups and military takeovers in Africa in the last decade.