The Economic Community of West African States (ECOWAS) has imposed sanctions on Niger and said it could authorise the use of force if the coup leaders do not reinstate elected president Mohamed Bazoum within a week from last Sunday.
"The military option is the very last option on the table, the last resort, but we have to prepare for the eventuality," said Abdel-Fatau Musah, ECOWAS Commissioner for Political Affairs, Peace and Security.
"There is a need to demonstrate that we cannot only bark but can bite," he told reporters in Abuja.
The delegation to Niger is being led by former Nigerian military leader Abdulsalami Abubakar, who arrived on Wednesday to start talks with the junta, Musah said.
The junta in Niger is led by the former head of Bazoum's presidential guard, General Abdourahmane Tiani, who shut Bazoum in his palace last Wednesday and later declared himself head of state.
ECOWAS has struggled to contain a democratic backslide in west Africa as member states Mali, Burkina Faso and Guinea have also had coups in the last two years, along with an attempted coup in Guinea-Bissau.
The regional bloc has taken its hardest line yet on Niger, prompting Mali and Burkina Faso, also ruled by juntas, to say that any intervention in Niger would be considered a declaration of war against them too.
Niger is a key international ally in a fight against Islamist insurgents, and the coup has been condemned by foreign powers who fear it could allow the militants to gain ground.
"The UK very much welcomes ECOWAS' actions and (they) are indeed decisive actions with a strong commitment to democracy," United Kingdom Foreign Secretary James Cleverly said after meeting Nigerian President Bola Tinubu in Abuja on Wednesday.
The coup leaders announced overnight they had reopened Niger's land and air borders with Algeria, Burkina Faso, Mali, Libya and Chad after closing them last week.
The reopened land borders are mainly in remote desert areas.
Niger's key entryways for trade and commerce remain closed due to sanctions imposed by ECOWAS.
France and Italy are removing their citizens from Niger amid growing fears of conflict.
The first military planes carrying mostly European citizens landed in Paris and Rome on Wednesday.
"Yesterday and today, with the help of our French friends, we were already able to fly more than 40 Germans out of Niger," German Foreign Minister Annalena Baerbock said, adding there would be more flights on Wednesday.
"It is also with this same unity and determination that we, as the European Union, support international efforts to restore constitutional order in Niger," she said.
France said it had helped more than 350 French people leave so far.
France, the United States, Germany, and Italy have troops in Niger on counterinsurgency and training missions, helping the army to fight jihadist groups.
Any European military intervention to restore democracy must be ruled out, as it would be "perceived as a new colonisation," Italian Foreign Minister Antonio Tajani said.
Niger is the world's seventh-biggest producer of uranium, the radioactive metal widely used for nuclear energy and treating cancer.
The European Union Commission said earlier this week that it had sufficient inventories of natural uranium to mitigate any short-term supply risks.
