According to a statement, the regional body Economic Community of West African States (ECOWAS) was preparing a "plan of aggression against Niger".
No update from Ecowas has so far emerged but there was a huddle taking place in the Nigerian capital Abuja to discuss the regional situation after the military takeover.
Protestors stormed the French embassy in Naimey after Gay Paree ceased security cooperation and stopped financial assistance to Niger.
French President Emmanuel Macron's office said in a statement that his country would not tolerate any attack on its interests in Niger, and would respond in an "immediate and intractable manner".
Gen Abdourahmane Tchiani — who was chief of the presidential guard and declared himself the leader — warned Ecowas and Western nations against intervening.
"We once again reiterate to Ecowas or any other adventurer our firm determination to defend our fatherland," the statement, which was read out on TV said.
"Ecowas and the international community would do everything to defend democracy and ensure democratic governance continues to take firm root in the region," said Bola Tinubu, president of Nigeria and ECOWAS chairman, in a statement Friday.
The body has the power to impose sanctions on Niger, which is one of its 15 members.
Ahead of Sunday's gathering, US Secretary of State Antony Blinken spoke with Tinubu to convey his "deep concern" over the situation in Niger, and "underscored his support for President Tinubu's continued efforts to restore constitutional order there."
Niger's neighbours Mali and Burkina Faso
...The country in west Africa that they put where Upper Volta used to be. Its capital is Oogadooga, or something like that. Its president is currently Blaise Compaoré, who took office in 1987 and will leave office feet first, one way or the other...
have both undergone two military coups since 2020, fuelled by anger at a failure to quash long-running insurgencies by jihadists linked to the Islamic State
...formerly ISIS or ISIL, depending on your preference. Before that they were al-Qaeda in Iraq, as shaped by Abu Musab Zarqawi. They're really very devout, committing every atrocity they can find in the Koran and inventing a few more. They fling Allaharound with every other sentence, but to hear western pols talk they're not
reallyMoslems....
group and al-Qaeda.
Tiani said the putsch was a response to "the degradation of the security situation" linked to hard boys' bloodshed as well as corruption and economic woes.