The meeting — originally scheduled for Saturday but then postponed — came after ECOWAS leaders last week approved deployment of a "standby force to restore constitutional order" in Niger, whose president was ousted on July 26.
Their summit, held in the Nigerian capital of Abuja last Thursday, also reaffirmed the bloc's preference for a diplomatic outcome.
President Mohamed Bazoum's election in 2021 was a landmark in Niger's history, ushering in the country's first peaceful transfer of power since independence from France in 1960.
His ousting unleashed a shock wave around West Africa, where Mali and Burkina Faso — likewise battered by a jihadist insurgency — have also suffered military takeovers.
The Economic Community of West African States (ECOWAS) gave Niger's military rulers a one-week ultimatum on July 30 to restore Bazoum or face the potential use of force, but the deadline expired without action.
Analysts say military intervention would be operationally risky and politically hazardous, given divisions within ECOWAS ranks and domestic criticism.
Niger's military regime has sent mixed signals since the crisis erupted.
At the weekend, the coup leaders said they were open to a diplomatic push after their chief, General Abdourahamane Tchiani, met with Nigerian religious mediators.
Those talks came after the ECOWAS military meeting in Ghana was postponed for "technical reasons."
But on Sunday night, Niger's rulers declared they had gathered sufficient evidence to prosecute Bazoum for "high treason and undermining internal and external security."
The legal threat was angrily condemned by ECOWAS, which called it a contradiction of the regime's "reported willingness" to explore peaceful means. Washington said it was "incredibly dismayed."
The row overshadowed talks under African Union (AU) auspices that began on Monday in Addis Ababa, bringing together representatives from the regime and ECOWAS.
A landlocked nation in the heart of the arid Sahel, Niger is one of the world's poorest and most turbulent countries.
Bazoum, 63, survived two attempted coups before being ousted, in the fifth putsch in the country's history.
His ousting deals a huge blow to French and U.S. strategy in the Sahel.
France refocused its anti-jihadist operations on Niger after withdrawing from Mali and Burkina Faso last year following a bust-up with their juntas.
International concern is mounting for the state of Bazoum, his wife and son, who have been detained at the president's official residence since the coup.