What you need to know:
- Primarily a Congolese Tutsi militia, the M23 is one of more than 120 armed groups active in conflict-torn eastern DRC.
Villagers dug graves for the two boys among banana trees, for they were too afraid of the fighting in the area to take the pair to the cemetery for burial.
Germain, aged six, and Isaac, seven, died when a shell hit their village school in eastern Democratic Republic of Congo (DRC) on June 10.
Their bodies were so shattered by the blast that their remains had to be buried immediately, their relatives said.
Biruma, a village in North Kivu province, found itself in the middle of clashes between government forces and the resurgent M23 rebel group.
Weeks of violence have grown into a diplomatic faceoff between the DRC and its neighbour Rwanda.
The DRC accuses Rwanda of abetting the rebels -- a charge that Rwanda denies -- and both countries accuse the other of carrying out cross-border shelling.
In remote villages like Biruma, some 50 kilometres (30 miles) north of the provincial capital Goma, innocent people are caught in the crossfire.
"We are tired of war, let them give us peace so that my eight other children can live," said Isaac's mother Sifa, 29, sitting outside her home.
Germain's father, Joseph Nziyunvira, said he wanted rich countries "to help Congo so that justice is done and the guilty are punished".
The Congolese army blamed Rwanda for the school shelling, which also wounded another boy, in an attack it called "both a war crime and a crime against humanity".
Nziyunvira too blames Rwanda, but he says he is neither "soldier nor politician".
"I am a farmer... but my son was killed suddenly. And we're still afraid".
'We live in fear'
Primarily a Congolese Tutsi militia, the M23 is one of more than 120 armed groups active in conflict-torn eastern DRC.
It briefly captured Goma in 2012 but a joint offensive by UN troops and the Congolese army quelled the rebellion.
But the M23 took up arms again in November last year after accusing the Congolese government of failing to respect a 2009 agreement under which the army was to incorporate its fighters.
The latest fighting has sharply worsened relations between the DRC and Rwanda, which have been strained since the mass arrival in the eastern DRC of Rwandan Hutus accused of slaughtering Tutsis during the 1994 Rwanda genocide.
On Tuesday, the Congolese government said it condemned "the participation of the Rwandan authorities in supporting, financing and arming this rebellion" and promised to defend "every inch" of its territory.
Congolese forces have stepped up patrols across North Kivu, but fear of attacks remains a constant.
Floribert Hakizumwami, Biruma's village chief, said people had started sleeping outside since the shelling.
"We live in fear," he said.
In the neighbouring village of Katale, shells pierced the tin roof of a school and destroyed two classrooms.
Elisabeth Nsengiyunva, carrying bags and trailed by her children and sheep, walked along the road outside of Goma.
She said she had heard that the rebels were coming "to kill us all."
"I don't have a given destination, I just have to save my life," she said.