DJUGU, Democratic Republic of Congo — The resource-rich but deeply troubled Democratic Republic of Congo is the site of some of Africa’s longest-running conflicts — and the world’s largest peacekeeping mission.

About 20,000 United Nations peacekeepers operate in Congo, Africa’s second most populous country, trying to keep its residents safe from the hundreds of armed groups that hide in and strike from its hills, especially in its east.

Violence is nothing new here, but a recent wave of brutal fighting has broken out in the province of Ituri, on the border with Uganda, raising concerns about a humanitarian catastrophe. More than 260 people have died and more than 200,000 have fled their homes since December in a conflict started by a scuffle between youths from two local ethnic communities, the Lendu and Hema.

The United Nations peacekeeping mission announced this past week it had discovered five suspected mass grave sites near some of the villages attacked in February and March, when the violence peaked.

The Lendu and Hema in Ituri live as neighbors, marry one another and speak the same language — but they also share a history of bloody conflict.

A dispute over land, nearly 20 years ago, escalated, and Ituri became the epicenter of a major regional war, involving foreign neighbors like Rwanda and Uganda, which backed different militias in their own battles for influence in Congo. Some of those foreign-backed militia leaders later became the first men convicted of war crimes at the International Criminal Court.

Ituri has been largely peaceful for more than a decade, but local grievances — especially over land — have never been resolved.

“Actually, they got worse because of the war,” said Séverine Autesserre, a Congo specialist at Barnard College in New York. New land conflicts popped up between people who had fled the violence and those who moved onto their abandoned land. “On top of that, you had grievances about what happened during the war: ‘You killed my family; you kicked me out.’”

Although many of the victims of the attacks this year have been Hema, experts say this new wave of violence does not follow the usual pattern of ethnic killings and reprisals.

“It’s not an ethnic conflict in the sense that these are not people who have always hated each other and are just going to kill and maim and rape each other because one of them is Lendu and one of them is Hema,” Ms. Autesserre said.

In some cases, the attackers are men who survivors say were speaking languages from other regions. In other cases, the attackers are Lendu, but no one knows what their motives are — and no one knows who is supporting them.

Many of the attacks were coordinated and carried out with new weapons and expensive communications equipment, suggesting the fighters have powerful backers who may be looking to exploit the animosity between the two ethnic groups for their own purposes.

One popular explanation for the violence is that President Joseph Kabila and his allies want to destabilize the region in order to postpone national elections scheduled for December. Mr. Kabila’s term expired two years ago, but he has illegally remained in office, and it is widely suspected his government may want to delay elections for a third year.

Although Corneille Nangaa, the top elections official in Congo, said in early February that the unrest in Ituri could affect the elections calendar, regional experts say there is no proof the government is behind the violence. Another theory is that foreign actors may have renewed interest in the resources in the region.

“There’s no one narrative that explains it all,” said Ida Sawyer, the Central Africa director for Human Rights Watch. “We can say with certainty that, at a minimum, violence is part of the system Kabila presides over and profits from.”

What is clear is the level of destruction. By mid-March, armed men had set at least 120 villages on fire, looting and raping during their attacks, according to the United Nations.

The attackers tried to kill anyone they found in the villages, survivors said, but most residents were able to escape.

Grace Mave, 12, barely survived the attack on her village, not far from Bunia, the provincial capital, in mid-February. Grace saw armed men rape and kill her pregnant mother.

The men also killed Grace’s siblings, ages 3 and 4, and they chopped off her left hand and slashed the back of her head with a machete. For nearly 24 hours, she lay on the ground, pretending to be dead, afraid the men might return.

Along the shores of Lake Albert, 42 people were killed in three villages in a single day last month, less than an hour after the Congolese military had finished patrolling the nearby waters, according to survivors, who blame the army for failing to protect them.

According to a report released Wednesday by the United Nations peacekeeping mission, Congolese security forces responded to only 10 of 70 attacks between February and mid-March.

The situation in the area has grown calmer, at least for the moment.

Etienne Unega Ege, Ituri’s interior minister, said on Thursday that the security forces had stabilized the area, and officials are encouraging people to leave the displacement camps.

But few of the displaced feel safe enough to go home.

Singo Lodja, 54, living in Bunia’s biggest displacement camp, said he was worried that the attackers were still armed. “We can’t go back,” he said.

Jean Bosco Lalo, a civil society leader in Ituri, said that as long as the killers and their motives remain unknown, Ituri will remain on edge.

“The people are hopeless,” he said.