Cameroonian refugees are seen in N Djamena on December 9, 2021. AFP
Violence broke out in the border village of Ouloumsa on Sunday in a dispute between herders, fishermen and farmers over dwindling water resources, the UNHCR said in a statement issued from Geneva.
It then spread to neighbouring villages, 10 of which have been burned to the ground.
The clashes have displaced thousands inside the country, "forcing more than 30,000 people to flee to neighbouring Chad," the UNHCR said.
"At least 22 people have been killed and 30 others seriously injured during several days of ongoing fighting."
The violence is unfolding in Logone-Chari in Cameroon's Far North region -- the tongue of land that lies between Nigeria to the west and Chad to the east.
The UN figures for those seeking refuge, and the death toll, are far higher than numbers given earlier by other sources.
The Chadian Red Cross said on Thursday there were at least 3,000 refugees, although the number was likely to grow, while the Cameroonian authorities said at least four had died.
Eighty percent of the new arrivals are women, including many who are pregnant, and children, the UNHCR said.
They have found refuge in the Chadian capital N'Djamena and villages along Chad's bank of the Logone River.
The UNHCR said at least 10,000 have fled to N'Djamena from Kousseri, a town of 200,000 people whose cattle market was destroyed in the fighting.
Cameroonian officials say two of the parties in the conflict are fishermen of the Musgum community and ethnic Arab Choa cattlemen.
Several thousand people have fled into the forest of Farcha, on the edge of N'Djamena, bringing with them mattresses and other possessions, an AFP reporter saw.
"What we witnessed was awful," Rahma Ahmat, a 55-year-old woman, said on Thursday. "I saw someone who was burned -- I was terrified."
Hadjime, a man aged 35, said he had taken part in the fighting with the Musgum.
"I fled because I was quickly overwhelmed by the situation and I didn't know where my child was," he said.
A bout of violence between herders and fishermen in August led to 45 deaths and an influx of at least 10,000 people into Chad.
As in the latest incident, the fighting began over management and access to water, the Cameroonian authorities say.
The UNHCR noted that friction in the region had been exacerbated by "the climate crisis."
"In recent decades, the surface of Lake Chad -- of which the Logone River is a main tributary -- has decreased by as much as 95 per cent," it said.
"Fishermen and farmers have dug vast trenches to retain the remaining river water so they can fish and cultivate crops. But the muddy trenches are trapping and sometimes killing cattle belonging to the herders, sparking tension and fighting."
Violent conflict between ethnic groups is relatively rare in Cameroon compared to Chad and Nigeria, where fighting over resources between semi-nomadic herders and sedentary farmers is frequent.
Chad, a country of around 17 million inhabitants, shelters a million refugees or internally displaced people (IDPs).
Chadian junta leader Mahamat Idriss Deby Itno on Wednesday issued a statement to say the situation was "worrying" and appealed to international donors to help the arrivals.
Cameroon hosts more than 1.5 million, many of them sheltering from jihadist violence in Nigeria.