Mr Ibn Chambas said the coup leaders were keen to return to normal duties
The leaders of the military coup in Niger have promised a "return to constitutional order", three days after overthrowing President Mamadou Tandja.
A spokesman for the coup leaders said they would draft a new constitution and hold elections, but did not say when.
Mr Tandja - seized during a cabinet meeting on Thursday - was being held at a house in the capital Niamey, he said.
Delegations from the UN and the West African regional body Ecowas have been in Niger for meetings with the junta.
In the third coup in the West Africa in the last 18 months, troops stormed the presidential palace in Niamey during a cabinet meeting, seizing Mr Tandja and his ministers before announcing that they were suspending the constitution and dissolving all state institutions.
Calling themselves the Supreme Council for the Restoration of Democracy, the coup leaders promised to turn Niger into an example of "democracy and good governance" and save its people from "poverty, deception and corruption".
A senior army officer, Col Salou Djibo, was named head of a military government.
After discussions on Sunday with Ecowas Commission President Mohamed Ibn Chambas, a spokesman for the military authorities said a new constitution would be created to replace the one amended in August that abolished limits on presidential terms of office.
Col Djibrilla Hima Hamidou compared Thursday's coup to the country's last one in 1999, when the then military leader, Col Ibrahim Bare Mainassara, was assassinated but civilian rule was restored in a year.
"This is not an army with a putschist tradition, that is not the case," he told reporters. "In 1999 we had a similar situation. We gave power back and we had 10 years of stability.
"We are going to do the same thing."
Population 14 million, 61% live on less than $1 a day
Huge reserves of uranium, Chinese firms digging for oil
History of coups, assassinations and on-off rebellion by nomadic Tuareg people in the north
Source: World Bank
Col Hamidou said the president was being held in a house in the grounds of the presidential palace and that the Red Cross would be allowed to visit him.
"Mr Tandja is in a service quarters of the presidency and is being kept in very good conditions."
"For now, we are taking care of his security and his health."
By Sunday, all but three ministers - Prime Minister Ali Badjo Gamatie, Interior Minister Albade Abouba and Finance Minister Ali Mahaman Lamine Zeine - had been released from house arrest, he added.
Mr Ibn Chambas said the coup leaders had told him that they intended to return Niger to democratic rule and then return to their normal military duties.
"They have assured us there will be an opening for everyone here in Niger, for an inter-Nigerien dialogue," he said.
"It is this process that will lead to a new constitution and credible elections. They said they want a short transition that ends as soon as possible, but it is also the political dialogue that will define the timetable."
The BBC's Caspar Leighton in Niamey says the city remains calm and people are going about their business as normal.
On Saturday, thousands of people staged a demonstration in support of the coup, many shaking the hands of soldiers on the streets.