At least 36 civilians were killed when a military plane bombed a funeral party in a Niger border village, the government said, in an incident its deputy mayor blamed on the Nigerian air force.
The air crew was likely to have mistaken the villagers, who had gathered near a mosque, for Boko Haram militants, Niger military sources in the nearby town of Bosso said.
The Nigerian military did not respond to Reuters' requests for comment about Tuesday's incident, into which the Niger government said it had launched an inquiry.
"Yesterday ... an unidentified plane dropped a bomb on the village of Abadam-Niger ... while the population was assembled near a mosque," the government said in statement read on state-owned television. "The provisional toll is 36 dead and 24 wounded."
The government decreed three days of national mourning.
"Two planes flew over Abadam on Tuesday afternoon. For me, with that very visible green color, they were from Nigeria," Abadam's deputy mayor, Youram Ari, told Niger television station Labari late on Wednesday.
Abadam lies on the border with Nigeria around 8 miles southwest of Bosso, where thousands of soldiers from Chad and Niger are massed in preparation for operations against Boko Haram.
Abadam is next to a Nigerian village of the same name and military sources in Bosso earlier said the bomb had fallen in Nigeria.
Boko Haram, which is seeking to carve an Islamic emirate out of northeastern Nigeria, killed an estimated 10,000 people there last year, and it is now expanding its zone of operations across the region's borders.
Nigeria, Chad, Cameroon, Niger and Benin are preparing a 8,700-strong force to combat the militants.