He said the current death toll of 95 would likely rise as more bodies were being found.
Fighting has skyrocketed in recent years between the two ethnic groups, who have a long history of disputes.
The conflict has been driven by suspicions on the Dogon side that the Fulani are collaborating with jihadi groups in attacks on Dogon villages aimed at stopping the residents working the land.
"The growing unrest and subsequent violence reported in the center of the country are characterized by killings, enforced disappearances, and burning of villages, on an appalling scale", said Barry. "No one was spared - women, children, elderly people".
The U.N. Security Council's meeting this month on Mali to discuss the renewal of what has become the world's deadliest active U.N. peacekeeping mission should focus on the protection of civilians, the rights group added. The Fulani are primarily cattle breeders and traders, while the Bambara and Dogon are traditionally sedentary farmers.
In the bloodiest raid, about 160 Fulani villagers were slaughtered on March 23, at Ogossagou, near the border with Burkina Faso, by suspected Dogon hunters.
Fulanis had "caused 63 deaths" among civilians in the Mopti region since that date, the United Nations added.
United Nations peacekeepers provided air support to the Malian government so they could prevent further attacks, UN spokeswoman Eri Kaneko told reporters in NY.
"The Government of Mali presents its deepest condolences to mourning families and assures all measures will be taken to arrest and punish the authors of this carnage", the communications ministry said in a statement. "Unless these concerns are immediately addressed, there is a high risk of further escalation of the situation in which atrocity crimes could be committed".
"Reinforcements are now deployed in the sector and carrying out a wide-ranging search", it said in a statement.
French forces intervened in Mali, a former French colony, in 2013 to push back a jihadist advance from the north but the militants have since regrouped.
A peace agreement signed in 2015 by the Bamako government and armed groups aimed at restoring stability.
Jihadi preacher, Amadou Koufa, led a group of Fulani and joined the Al-Qaeda-linked Group to Support Islam and Muslims (GSIM), headed by Iyad Ag Ghaly.