Since the July 26 power grab shocked the region, the defiant junta has rebuffed diplomatic overtures and ignored a deadline last Sunday from the Economic Community of West African States (ECOWAS) to reinstate ousted president Mohamed Bazoum.
The meeting in the Nigerian capital, Abuja, could prove a pivotal moment in the stand-off.
The bloc's leaders are expected to agree on next steps, which could include military intervention - something an ECOWAS official has said would be a last resort.
Envoys of the Nigerian president and ECOWAS chair Bola Tinubu met coup leaders in the capital, Niamey, on Wednesday, offering a glimmer of hope for dialogue after previous missions were spurned.
Any escalation would further destabilise West Africa's Sahel region, one of the world's poorest, where a long-running Islamist insurgency has displaced millions and stoked a hunger crisis.
The coup was triggered by internal politics but it has evolved into an international entanglement, with ECOWAS, the United Nations and Western countries putting pressure on the junta to stand down, while military governments in neighbouring Mali and Burkina Faso have vowed to defend it.
The summit takes place as Niger accused France of violating its airspace, attacking a military camp and freeing "terrorists" to undermine the country.
Paris has denied the charges.
Army officer Amadou Abdramane, speaking for the coup leaders, made the allegations in a video statement without providing evidence, stoking tension ahead of Thursday's meeting.
"What we are seeing is a plan to destabilise our country," Abdramane said.
France's foreign ministry rejected the accusations, saying its aircraft was operating under an existing agreement with Niger forces and its troops were in the West African nation at the request of legitimate authorities.
The party of Niger's deposed president Mohamed Bazoum accused the junta of keeping him and his family in "cruel" and "inhumane" detention at the presidential residence.
In a statement calling for a nationwide mobilisation to save them, the PNDS-Tarayya party said the Bazoums had no running water, no electricity and no access to fresh goods or doctors.
State Department spokesperson Matthew Miller said the US was greatly worried about Bazoum's safety and was still looking for a reversal of the military takeover.
"We continue to engage with our partners in the region," he told a briefing
"We continue to engage with other governments."
US troops are in Niger along with French, Italian and German forces as part of international efforts to combat Islamist insurgents devastating the Sahel region under agreements with the now-deposed civilian government.
The rhetoric against former colonial power France has been a feature of coups in the region in the past two years, including in Mali and Burkina Faso, whose army rulers are strongly backing the generals now in charge in Niamey.
The junta has revoked military pacts with France, but Paris has rejected that decision, saying it was not taken by Niger's legitimate authorities.
