West Africa's main regional bloc on Friday said it had agreed an undisclosed "D-Day" for a possible military intervention to restore democracy in Niger if diplomatic efforts fail, stressing that it would not hold endless dialogue with the defiant junta.

The comments came at the end of a two-day meeting of West African army chiefs in Ghana's capital Accra, where they have been hashing out the logistics and strategy for a possible use of force in Niger. Economic Community of West African States (ECOWAS) has said such action would be a last resort.

"We are ready to go anytime the order is given," ECOWAS Commissioner for Political Affairs, Peace and Security Abdel-Fatau Musah said during the closing ceremony. "The D-Day is also decided, which we are not going to disclose."

He said a peaceful resolution remained the bloc's preferred option.

"As we speak we are still readying (a) mediation mission into the country, so we have not shut any door... (but) we are not going to engage in endless dialogue."

There was no immediate response from the junta.

Military officers deposed Nigerien President Mohamed Bazoum on July 26 and have defied calls from the United Nations, ECOWAS and others to reinstate him, prompting the bloc to order a standby force to be assembled.

"We've already agreed and fine-tuned what will be required for the intervention," Musah said, declining to share how many troops would be deployed and other strategic details.

Most of its 15 member states are prepared to contribute to the joint force excepting those also under military rule - Mali, Burkina Faso and Guinea - and Cape Verde, according to the bloc.

ECOWAS has taken a harder stance on the Niger coup, the wider region's seventh in three years, than it did on previous ones. The credibility of the bloc is at stake because it had said it would tolerate no further such overthrows.

"The decision is that the coup in Niger is one coup too many for the region, and we are putting a stop to it at this time, we are drawing the line in the sand," Musah said.

Any intervention would spell further turmoil for West Africa's impoverished Sahel region, which is already battling a decade-old Islamist insurgency and a deepening hunger crisis.

Niger also has strategic importance beyond West Africa because of its uranium and oil reserves and role as a hub for foreign troops involved in the fight against the insurgents linked to al Qaeda and Islamic State.

Meanwhile, diplomatic efforts continue. The United Nations special envoy for West Africa and the Sahel, Leonardo Santos Simao, met with the junta's Prime Minister Ali Mahamane Lamine Zeine on Friday.

Simao said in comments broadcast on Niger's state television that he wanted to listen to the junta's point of view "to study together a way for the country to return as quickly as possible to constitutional normality and legality too. We are convinced that it is always possible with dialogue."

Reuters