Nigerian terror group Boko Haram has killed at least 30 civilians in Niger, which is part of a regional coalition aimed at tackling terrorism in northern Nigeria.
The militants attacked two villages in southern Niger's Diffa region, two security sources told news agency AP.
Diffa has already been attacked in the past by the militants. In February, a huge explosion in the town injured several people.
The latest attack follows twin suicide bombings in Chad's capital N'Djamena, in which dozens of people were killed earlier in June.
The Chad government blamed Boko Haram for the bombing. The group has not claimed responsibility for the attacks, but urged Chad to withdraw its troops from Nigeria, warning imminent attacks would take place otherwise.
In the aftermath of the attack, Chad's Communications Minister Hassan Sylla Bakari blamed the attacks on the Nigerian terror group and said: "Boko Haram is making a mistake by targeting Chad. These lawless terrorists will be chased out and neutralised wherever they are."
A few days later, Chad carried out air strikes in northern Nigeria destroying six Boko Haram camps.
Who are Nigeria's Boko Haram terrorists?
Boko Haram (recently renamed Iswap) fights against Western influence in Nigeria and aims to impose its version of Sharia law on the country. The group declared an Islamic caliphate in Gwoza, along the Cameroon border, in August 2014.
Boko Haram has raided several cities in the north of the country in a bid to take control of more land.
Three states − Adamawa, Borno and Yobe − have been under a state of emergency since May 2013, due to Boko Haram's attacks.
The group has killed at least 2,600 people since the beginning of 2015. More than 180 have been killed since the beginning of June.
Government spokesman Mike Omeri told IBTimes UK that the Nigerian government is aware of the attack in Niger, but cannot say whether it has been carried out in retaliation to Chad's participation in the regional joint mission.
"There were attacks, such as in Cameroon, even before [the regional coalition]. This is something we have always tried to prevent."
The Nigerian government is being aided by mercenaries and troops from Chad, Benin, Niger and Cameroon in its offensive and has scored some successes since the military co-operation started in February.
Newly elected President Muhammadu Buhari, a former military chief, vowed Nigeria would do anything it could to defeat the deadly insurgence and find some 220 girls who were kidnapped by Boko Haram in Chibok, Borno state, in April 2014.
The US announced it would provide $5m (£3.1m) to the regional force in addition to the $34m (£21.6m) already provided to Chad, Niger and Cameroon for logistics and other equipment.