BAMAKO (Reuters) - Malian forces involved in the fight against Islamist militants in the West African nation executed 12 civilians in retaliation after a soldier was killed in an attack in May, the U.N. mission in Mali said on Tuesday.

Mali’s fight against jihadist groups in the center and north of the country has been marred by a series of alleged abuses, some of which the government has acknowledged.

Those abuses, as well as tit-for-tat attacks by rival ethnic groups, have fueled surging violence across vast swathes of Mali, raising doubts about the government’s ability to organize a presidential election scheduled for July 29.

The U.N. mission in Mali, MINUSMA, said in a statement that its investigation had concluded that Malian troops from the G5 Sahel - a joint task force with Niger, Chad, Burkina Faso and Mauritania - “summarily and/or arbitrarily executed 12 civilians at the Boulkessy cattle market” on May 19.

It said that the report has been handed to Mali’s government and that Mahamat Saleh Annadif, the head of MINUSMA, “calls on the Malian authorities to ensure that the investigation underway is carried out as quickly as possible”.

There was no immediate comment from Malian authorities.

Mali’s government said last week that some of its soldiers were implicated in gross rights violations after the discovery of mass graves in central Mali. Rights groups have alleged widespread abuses, but the government has rejected those claims.