Bamako: Twenty-five bodies were found in central Mali after the army carried out a sweep in the unstable region, sources said on Monday, adding to concern about abuse by security forces in their fight against extremists.

A non-government organisation (NGO) called Kisal, which campaigns for the human rights of pastoral communities, said in a statement "25 bodies" had been found in three mass graves.

It provided a list of 18 names of people who, it said, had been killed.

The grim discovery was made after 25 people from the Fulani ethnic group, who are predominantly herders, were picked up last week by the army in the localities of Kobaka and Nantaka, Kisal said.

Separately, Oumar Diallo, a member of the Fulani association Tabital Pulaaku, said in the main regional town of Mopti that the first grave had seven bodies, the second held 13 and there were five others more in the third.

Central Mali is a vast area where the state is near-absent and extremists, blamed for exacerbating the dispute, roam with little constraint.

The armed forces are facing increasing accusations of arbitrary arrests and extra-judicial killings in their fight against the insurgents.

A source at the defence ministry told AFP that an inquiry had been opened but denied "these accusations of summary executions".

A ministry spokesman also denied there had been any abuse.

"The zone is dangerous," he said, explaining "terrorists and unidentified armed men" had been in the area.

On May 19, the army said three Malian soldiers and 12 "terrorists" were killed in fighting at an army camp near the border with Burkina Faso. But locals alleged the dead were all civilians and the army later put out a new statement that spoke of 12 "people" killed.

A resident of Nantaka named Hama Kelly said that troops arrested every person they came across as soon as they arrived in the village.

"They took their mobile phones and identity cards. Afterwards, people who were [members of the] Songhai [ethnic group] were released but all the Fulani were kept behind," Kelly said.

The governor of the Mopti region, General Sidi Alassane Toure, declined to make any comment when approached by AFP.

"An army unit is in the area, I am awaiting its return to find out the situation," he said.

Tensions and violence have intensified in the Mopti area over the past three years, featuring clashes between Fulani herdsmen and sedentary farmers from other ethnic groups who accuse the pastoralists of colluding with extremists.

Extremists linked to Al Qaida 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.

The insurgents have morphed into nimbler formations operating in rural areas, sometimes winning over local populations by providing basic services and protection from bandits.