General Abdourahamane Tiani, the former head of Niger's presidential guard who shut President Mohamed Bazoum in his residence last week and declared himself head of state, vowed not to bow to pressure in a speech on state television on Wednesday night.
The Economic Community of West African States (ECOWAS), whose defence chiefs were meeting in the Nigerian capital of Abuja, has unleashed sanctions and said it could authorise the use of force if soldiers did not restore Bazoum to power by Sunday.
Tiani called the sanctions "inhumane" and said he rejected any foreign interference but was open to dialogue within the country.
"We have always listened with humility to all the parties concerned and are open to dialogue so that ... together we can create the conditions for a peaceful transition that should lead to elections within a relatively short and reasonable time," he said.
He did not give any timeline.
ECOWAS has struggled to contain a democratic backslide in West Africa and had vowed coups would no longer be tolerated after military takeovers in Mali, Burkina Faso and Guinea and an attempted coup in Guinea-Bissau in the past two years.
The defence chiefs aim to draw up a plan for eventual military intervention to restore constitutional order in Niger, according to a statement from Ivory Coast's National Security Council.
Mali and Burkina Faso - also ruled by juntas - have said that they would consider such an intervention a "declaration of war" against them too, and would withdraw from ECOWAS and come to Niamey's defence.
Tiani sent a general to both countries on Wednesday to shore up support.
"In co-ordination with our brothers in Burkina Faso, we have decided to undertake a number of activities to secure our populations and secure our two countries," Niger General Salifou Mody said in a statement issued by Burkina Faso's government after his meeting in Ouagadougou.
There are signs the regional sanctions are starting to have an impact.
Nigeria cut power supplies to Niger, while Nigerien truckers were stranded in limbo by border closures.
Supporters of Niger's junta planned a march in the capital Niamey on Thursday to protest the sanctions and demand the departure of French troops stationed in Niger to help it fight an Islamist insurgency.
Thursday also marks the 63rd anniversary of Niger's independence from France.
United States President Joe Biden said in a statement marking the day that Nigeriens had the right to choose their own leaders.
Like the recent coups in neighbouring Burkina Faso and Mali, Niger's military takeover has come amid a growing wave of anti-French sentiment, with locals saying they want the former colonial ruler to stop interfering in their affairs.
Western countries are concerned Niger could turn towards Russia as an ally as some of its neighbours have done.
Niger has been an important Western ally in a fight against groups linked to al Qaeda and Islamic State, and the coup has been condemned by foreign powers who fear it could allow the militants to gain ground.
Niger is also the world's seventh-biggest producer of uranium, the radioactive metal widely used for nuclear energy and treating cancer.
The turmoil has prompted some European nations to evacuate citizens by plane.
On Wednesday, the US said it had ordered the evacuation of some staff and families from its embassy even though the mission remains open and senior staff continue working.
