The groups have also destroyed schools and churches and imposed restrictions based on their harsh interpretation of Islam
More than 420 civilians have been killed in jihadist attacks in western Niger this year and tens of thousands of people have fled their homes, Human Rights Watch (HRW) said on Wednesday.
The estimate comes after an 11-day trip in which members of the watchdog travelled to Niger to meet witnesses, traditional chiefs, local authorities, foreign diplomats and Nigerien rights activists.
"Armed Islamist groups appear to be waging war on the civilian population in western Niger," said Corinne Dufka, HRW's Sahel director.
"They have killed, pillaged and burned, leaving death, broken lives and destruction in their wake," she said in the HRW statement.
The groups have also destroyed schools and churches and imposed restrictions based on their harsh interpretation of Islam, the rights group said.
Nine attacks documented by HRW took place between January and July in towns and villages in the western regions of Tillaberi and Tahoua.
"Since 2019, this area has experienced a dramatic spike in attacks against military targets and, increasingly, civilians by armed Islamist groups allied to the Islamic State and, to a lesser extent, Al-Qaeda," HRW said in its report.
Among those killed were village chiefs, imams, people with disabilities and "numerous children," some executed after being ripped from their parents’ arms, it added.
"People who commit serious violations of the laws of war, including summary executions and torture, may be prosecuted for war crimes," HRW said.
According to official figures, 307 civilians were killed in the western region between January and March this year.
Atrocities have also been committed in southeast Niger by Nigerian jihadists from Boko Haram and the Islamic State West Africa Province (ISWAP).