By Aaron Ross

KINSHASA, June 22 (Reuters) -At least 12 people were killed in northeastern Democratic Republic of Congo in heavy firefights between the army and militia fighters on Thursday, and several students sitting exams were wounded in an explosion at a school, local activists said.

The fighting in and around the city of Beni between Congo’s army and what is believed to be a new coalition of armed groups, the National Movement of Revolutionaries (MNR), killed at least eight militiamen and four soldiers, said activist Teddy Kataliko.

The clashes, some of which occurred near the mayor’s office, broke out early Thursday morning but the army had driven back the militias by mid-afternoon, he added.

Gilbert Kambale, another local activist, told Reuters that at least 13 militiamen and three soldiers had died in Thursday’s fighting.

The mayor and a local army spokesman could not be immediately reached for comment on Thursday afternoon.

The fighting followed a breakout by more than 900 inmates, many suspected militiamen, from Beni’s main prison this month – one of a series of mass jailbreaks that have undermined security in Congo since President Joseph Kabila refused to step down at the end of his constitutional mandate in December.

Worsening security in the vast central African nation has raised fears of a return to the civil wars of the turn of the century that killed millions, most from hunger and disease, and sucked in more than half a dozen neighbouring countries.

Kataliko and Kambale also said unidentified assailants set off an explosive device at a local secondary school, wounding several students sitting exams. A hospital source said at least three students were injured in the blast.

Eastern Congo contains dozens of armed groups that prey on locals and exploit mineral reserves. Hundreds of civilians have died near Beni since October 2014 in a series of overnight massacres, mostly carried out with hatchets and machetes. It is still not clear who is responsible for most of the attacks. (Reporting by Aaron Ross; Editing by Richard Balmforth and Toby Davis)