Barlerin alleged on May 18 that government forces had carried out "targeted killings" and other abuses against militants demanding independence for two English-speaking regions.
"On the side of the government, there have been targeted killings, detentions without access to legal support, family, or the Red Cross, and burning and looting of villages," he said in a statement.
"On the side of the separatists," he also stressed, "there have been murders of gendarmes, kidnapping of government officials, and burning of schools".
The crisis began in 2016, when activists in the Anglophone minority stepped up a campaign for greater autonomy.
According to the International Crisis Group (ICG) think tank, "at least 120" civilians and "at least 43" security forces have been killed since the end of 2016.
The Anglophone minority accounts for about a fifth of the country's population of 22 million.
Biya has rejected the Anglophone demands, prompting radicals to make a full-blown but symbolic declaration of independence last October 1.
That was the starting point for armed clashes and a crackdown.
Barlerin, in his statement, called on both sides "to stop the violence immediately." Many Anglophones in the Northwest and Southwest regions complain of marginalisation in education, the judiciary and the economy and of having French imposed on them.
Yesterday, the UN's Office for the Coordination of Humanitarian Affairs (OCHA) said around 160,000 people had been internally displaced by the crisis and 20,000 had sought refuge in neighbouring Nigeria.
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