Helene Caux, UN High Commissioner for Refugees spokeswoman, said on Thursday: "According to information we have been given by villagers, there were at least 220 killed and many wounded just in the Kersi region," south of the town of Goz Beida.
"In the village of Djorlo, which we visited yesterday [Wednesday], there were 36 confirmed dead and 22 wounded."
Caux was taking part in a joint mission by staff from UN agencies.
At Djorlo, she added: "Shocked survivors were wandering around on a site that had been completely burned down and looted."
Residents of several villages where huts were still in flames told an AFP correspondent that the attacks had gone on into Wednesday.
The assaillants "wore military battle dress" and were armed with assault rifles, Caux said, reporting what she had heard from villagers. About 200 men took part in the raids "on horseback and accompanied by two pick-up trucks".
"These witness accounts lead us to think the attacks were organised."
Caux pointed out that the clashes took place only a few miles from camps for Darfur refugees and said the UN team had seen "an extremely shocking and desolate sight, these scenes remind us of the Darfur crisis".
In Geneva, Antonio Guterres, UN High Commissioner for Refugees, said on Thursday that his agency had "warned for months that the Darfur conflict threatens to destabilise the entire region".
UNHCR teams in southeastern Chad had already reported that as many as 220 people may have been killed in a string of attacks this week by armed men on horseback, with dozens wounded and out of reach of health centres.
At least 1,000 people are said to have fled their homes.