The official made the comments 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 that ECOWAS has said 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."
"We've already agreed and fine-tuned what will be required for the intervention," he said, emphasizing that it was still seeking to engage with the junta peacefully.
"As we speak we are still readying (a) mediation mission into the country, so we have not shut any door."
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.
Most of its 15 member states are prepared to contribute to the joint force except those also under military rule — Mali, Burkina Faso and Guinea — and tiny Cape Verde, an ECOWAS official said on Thursday.
Defense chiefs have declined to say how many troops will be deployed or other strategic details.
Any intervention would further destabilize West Africa's impoverished Sahel region, which is already battling a decade-old Islamist insurgency.
The United Nations on Friday slammed the generals who seized power in Niger on "a whim" and plunged the country further into misery, demanding that constitutional order be immediately restored.
"The very notion of freedoms in Niger is at stake," U.N. rights chief Volker Turk said in a statement.
"Generals cannot take it upon themselves to defy — at a whim — the will of the people," he said. "Rule-by-gun has no place in today's world."
He also described "a clampdown on civic space" in the country, pointing to allegations of intimidation against journalists and bans on international media outlets, as "very worrying."
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.
Information for this report came from Reuters and AFP.