The United Nations has expressed its deep concern about ongoing ethnic fighting in northeastern Democratic Republic of Congo, warning that the violence that has forced tens of thousands to seek shelter in neighbouring Uganda "may spread further".

More than 22,000 people fled clashes between Hema herders and Lendu farmers in Ituri province last week, according to the UN's refugee agency (UNHCR).

This marked a dramatic increase in the number of people crossing Lake Albert to reach Uganda in search of safety, bringing the overall figure since the start of the year to 34,000.

The latest influx follows what a UNHCR spokesperson called "the latest and biggest wave" in an outbreak of violence that started in the area in December.

"A number of local monitors have confirmed to us that more than 1,000 houses have been burned in Djugu, Ituri territory, which is the epicentre of the violence. The displaced say that groups of armed men set houses on fire and killed civilians," Andreas Kirchhof told Al Jazeera on Wednesday.

"It's a very worrying situation. There is fear the violence may spread further."

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UNHCR staff reported that several villages along Lake Albert had been entirely deserted.

A further 15,000 internally displaced persons arrived in Bunia, capital of Congo's Ituri province, in the past 10 days, a spokesperson for the UN Office for the Coordination of Humanitarian Affairs told Al Jazeera.

Yvon Edoumou said there were reports of several dozen deaths in Djugu, as well as homes being burned down.

Renewed conflict

The violence is thought to be the latest escalation in the ongoing conflict between the Hema herders and the Lendu farmers.

Due to a lack of access to the territory, it is difficult to verify who is perpetrating the violence and what caused the surge.

"What we can say is that the displaced people have been attacked by armed men, people they didn't know,” Kirchhof told Al Jazeera.

The governor of Ituri, Jefferson Abdallah Penembaka, told Reuters news agency that the Hema-Lendu conflict killed at least 30 people over two days in early February.

Traumatised

International aid group CARE also expressed concern over the refugee influx from Congo to Uganda, saying that the majority of fleeing women had experienced or witnessed gender-based violence on their way to Uganda.

In a statement on Wednesday, the organisation said the majority of refugees are women and children and that most of them are "heavily traumatised".

"People arrive exhausted, dehydrated, hungry and emotionally devastated. They are running for their lives," Delphine Pinault, CARE's country director for Uganda, said in the statement.

"We've heard stories of people waiting to cross from the other side who are spending the night in the lake for fear of being attacked."

The UNHCR warned that more people might lose their lives while making the dangerous lake crossing.

Four Congolese refugees are feared to have drowned after their boat capsized while crossing to Uganda on February 11.

Ethnic strife between the Hema and Lendu dates back to the 1970s. The communities were embroiled in a violent armed conflict between 1998 and 2003, in which tens of thousands were killed. In recent years, the two groups have maintained a low-level conflict with occasional flare-ups in violence.

Last year, conflict forced 1.7 million people across Congo to flee their homes.

In October 2017, the UNHCR said there were 3.9 million internally displaced persons in the country, and more than 600,000 refugees from Congo spread over 11 African countries.