In a separate incident in the same region, six Malian soldiers were killed on Tuesday when their vehicle struck a mine, the army said on Wednesday.

Malian soldiers and international forces are battling jihadists and criminal gangs in the north and centre, where an Islamist insurgency has taken root and the state is largely absent.

Nouhoun Sarr, the nephew of one of seven men killed on February 21 after their arrest by the military, said the authorities informed him on Wednesday morning of his uncle's fate.

"They called us to tell us our relatives were killed during the operation, without elaborating," he told AFP.

A UN report published this month found that "at least 20 percent" of recorded incidents in 2016 and in the first of half of 2017 that endangered civilian lives involved the Malian authorities -- essentially the security forces.

"According to our information, the seven civilians arrested on February 21 by the Malian army in Sokolo were killed by soldiers," said Yehia Ag Mohamed Ali, a former minister and member of the opposition Sadi party.

The government said in a statement on Wednesday that the army was "carrying out reconnaissance missions and searches in the Sokolo area, during which civilians unfortunately lost their lives.

"An enquiry has been opened," it added.

The army suffers losses on a weekly basis at the hands of jiahdists, and on Tuesday a lieutenant was among the six killed, an army statement said.

The soldiers were travelling in the Segou region when they "triggered an improvised explosive device" at 6:30pm (1830 GMT) on Tuesday, according to the statement.

Islamic extremists linked to Al-Qaeda took control of the desert north of Mali in early 2012, but were largely driven out in a French-led military operation launched in January 2013.

In June 2015, Mali's government signed a peace agreement with coalitions of armed groups. But the jihadists remain active, and large tracts of the country are lawless.