The cause was not immediately known, but the two helicopters might have collided with each other before crashing into the water, Defense Minister Minoru Kihara said.
Niger has been home to a major U.S. airbase in the city of Agadez, some 550 miles from the capital Niamey, using it for manned and unmanned surveillance flights and other operations.
The Treasury Department announced sanctions on two entities accused of fundraising for extremist West Bank settlers connected to violence against Palestinians.
The break in tradition does not sit well with the Association of Summer Olympic Committee, who said it undermines "the value of Olympism and the uniqueness of the games.
Paris police cordoned off an area around an Iranian consulate amid reports of a man threatening to detonate a bomb, but a suspect was quickly detained.