Burkina Faso and Mali have issued a joint statement saying that they would regard any military intervention in Niger as a declaration of war on them.
The junta in Guinea has also joined the call.
On Sunday the regional organisation Ecowas threatened to use force to reinstate Niger's elected President Mohamed Bazoum, who was deposed in a coup on Wednesday.
But Burkina Faso and Mali said in such a scenario they would come to the defence of their eastern neighbour. They would also quit Ecowas.
The warning from the two junta-led countries marks a significant twist that could escalate the volatile regional situation.
They said such a move would be disastrous and destabilising.
Both countries have severed ties with the West and embraced alliances with Russia. Like Niger, they were colonised by France and have been fighting extremists in the Sahel.