More than 300,000 people have been displaced this month by ethnic violence in Ituri province in the Democratic Republic of Congo, the UN says.
There are fears that the latest clashes - between Hema cattle-herders and Lendu farmers - could hamper efforts to combat an Ebola outbreak in the region.
The two communities have repeatedly fought over land and water in Ituri, a gold-rich region in the north-east.
The rivalry left thousands dead between 1997 and 2003, amid a wider conflict.
In 2012, a Hema warlord involved in the conflict, Thomas Lubanga, became the first person to be convicted by the International Criminal Court (ICC).
In 2017, a Lendu militia leader, Germain Katanga, became the first convicted war criminal to be ordered to pay damages to his victims by the ICC.
A total of six million people were killed during six years of war in DR Congo. Most of the deaths were from disease and malnutrition.
Several neighbouring countries were involved in the fighting, described by some observers as "Africa's world war".
At least 160 people have reportedly been killed in the latest violence in Ituri province. Most of the victims are believed to be from the Hema community.
The exact origins of the latest violence are not yet known. The displaced are being cared for by members of their own community, or in camps, where conditions are said by the United Nations refugee agency (UNHCR) to be "dire".
UNHCR spokesman Babar Baloch told journalists in Geneva that the agency was "gravely concerned" by the latest reports, and feared that "this escalation could engulf large parts of the province".
Mr Baloch said the agency had received reports of "large-scale displacement" in three of Ituri's five administrative territories - Djugu, Mahagi and Irumu.
"People are fleeing attacks and counterattacks in Djugu Territory, with reports of both communities forming self-defence groups and being involved in revenge killings," he said.
He added there had been reports of kidnapping, maiming and sexual violence, and that the UNHCR and other humanitarian agencies did not have access to most of the affected areas.
The displacement could endanger the response to an Ebola epidemic in the region, a spokesman for the UN's health agency, the WHO, told reporters.
Tarik Jasarevic said it was "more complicated" to monitor people on a daily basis for 21 days - the disease's incubation period - whenever there were large numbers of people on the move.
More than 1,400 people have been killed in the current Ebola outbreak - the second-largest in history. The outbreak has largely been confined to the eastern DR Congo, though cases have also been recorded in neighbouring Uganda.
About 10% of the latest cases and deaths were recorded in Ituri, Mr Jasarevic said.