President Mohamed Bazoum’s comments came as a team from the West African bloc, Ecowas, left Niger without meeting the leader of the junta that seized power, a delegation member said on Friday.
With two days remaining before a deadline set by Ecowas to release and reinstate the President or face possible force, Mr Bazoum wrote a plea published in The Washington Post, in which he said: “I write this as a hostage.”
The leader, who came to power in a democratic election in 2021, said Niger currently stands as “the last bastion” of respect for human rights in the region.
And he said that while the coup is a “tragedy for Nigeriens”, its success could have “devastating consequences” far beyond the country’s borders.
“With an open invitation from the coup plotters and their regional allies, the entire central Sahel region could fall to Russian influence via the Wagner Group, whose brutal terrorism has been on full display in Ukraine.
“Boko Haram and other terrorist movements will surely take advantage of Niger’s instability, using our country as a staging ground to attack neighbouring countries and undermine peace, safety and freedom around the world.
“They will ramp up their efforts to target our youths with hateful anti-western indoctrination, turning them against the very partners who are helping us build a more hopeful future.”
He said the US, the African and European unions, and the Economic Community of West African States have been “loud and clear”.
“This coup must end, and the junta must free everyone they have unlawfully arrested.”
He called on the US and the international community to help restore constitutional order.
“Fighting for our shared values, including democratic pluralism and respect for the rule of law, is the only way to make sustainable progress against poverty and terrorism. The Nigerien people will never forget your support at this pivotal moment in our history,” he wrote.
Kremlin spokesman Dmitry Peskov was responded to Mr Bazoum's call for the United States and the entire international community to intervene to restore constitutional order in his country on Friday.
"It is unlikely that the intervention of non-regional forces is capable of changing the situation for the better," Mr Peskov said. "We are monitoring the situation very closely, we are concerned about the tension in Niger, and we continue to favour a swift return to constitutional normality without endangering human lives."
The Ecowas delegation arrived in the capital Niamey on Thursday "but did not spend the night" as scheduled, nor meet with coup leader Abdourahamane Tiani or Mr Bazoum, a team member said.
The delegation was led by former Nigerian president Abdulsalami Abubakar and was initially due to meet Gen Tiani to present Ecowas's demands, according to the Nigerian presidency.
Regional powerhouse Nigeria holds the rotating presidency of Ecowas, which imposed sanctions and on Sunday gave the putschists a week to restore Mr Bazoum to power.
Nigerian President Bola Tinubu said the bloc would do its best to resolve the crisis amicably but Ecowas said it could resort to military intervention as a last resort.
Niger evacuation flights land in Europe - in pictures
Niger's junta meanwhile said it would meet “force with force”.
"Any aggression or attempted aggression against the State of Niger will see an immediate and unannounced response from the Niger Defence and Security Forces on one of [the bloc's] members", one of the putschists said on national television late on Thursday.
France completed its evacuation of Niger on Thursday, saying more than 1,000 people had left in the operation.
On Tuesday and Wednesday this week "our embassy in Niamey, the Ministry for Europe and Foreign Affairs’ Crisis and Support Centre and the Ministry for the Armed Forces’ Planning and Conduct of Operations Centre evacuated a total of 1,079 people, including 577 French nationals", the Foreign Ministry said in a statement.
The operation also involved citizens of 50 other countries, from areas including Europe, Africa, the US, and the Middle East.