At least 36 people were killed at a funeral ceremony when an unidentified plane bombarded a village on the Niger-Nigeria border, officials said.
Villages on both sides of the border are called Abadam, and there was initially some confusion over which side of the border was hit. Officials later on Wednesday said the attack happened in Niger.
"Yesterday ... an unidentified plane dropped a bomb on the village of Abadam-Niger ... while the population was assembled near a mosque," Niger's government said in a 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 colour, they were from Nigeria," Abadam's deputy mayor Youram Ari told Niger television station Labari.
A Nigerian military spokesman, however, denied involvement.
Air Commodore Dele Alonge told the AFP news agency: "It's not to my knowledge and there has not been any report from our people of such an incident."
An elected local leader in nearby Bosso said a similar air strike several days ago on the nearby village of Gamgara killed one person.
Abadam lies on the border about 13km southwest of Bosso, where thousands of soldiers from Chad and Niger are massed in preparation for operations against Boko Haram.
Niger is part of a joint regional force that is involved in ongoing operations against the armed Nigerian group. Niger has committed more than 700 troops to the regional force.
The border region has seen recent attacks and counter-attacks between the regional forces and Boko Haram.
Separately on Wednesday, Nigerian military sources claimed to have killed more than 300 Boko Haram fighters during an operation to recapture 11 towns and villages since the beginning of the week, the Reuters news agency reported.
"Weapons and equipment were also captured and some destroyed," defence spokesman Major-General Chris Olukolade said in a statement.
"However, two soldiers lost their lives while 10 others were wounded."
It was not possible to independently verify the military's statement.
Nigerian forces have in the past been accused of overstating enemy casualties while greatly understating their own and those of civilians caught in the crossfire.