Soldiers tried to attack a key military base early on Saturday morning, authorities say, but were thwarted.
Kevin Rideout, who was kidnapped outside his home in the capital last October, is now with US officials.
France says the coup leaders have no authority to order such an expulsion, as ties deteriorate rapidly.
Acting Deputy Secretary of State Victoria Nuland met Niger's military leaders for over two hours.
The government said armed attackers tried to seize the presidential palace overnight.
The vote is being hailed as the first democratic transition in the history of the coup-prone state.
