Brazilian authorities need to show "zero tolerance" in letting aggressors get away with such violence against native peoples, the UN office in the South American country said in a statement.
Brazilian police are probing the confrontation that occurred Sunday in northeastern Maranhao state.
Thirteen members of the Gamela tribe were hurt in the clash with ranchers which is believed to have been linked to land disputes.
Media reported that four of the ranchers were also wounded in the melee, which involved machetes.
The Brazilian news website G1 quoted local police chief Jorge Pacheco saying both sides were also carrying firearms.
"We're still at the beginning of the investigation, but initial indications are that there was aggression on both sides, as both sides had firearms," Pacheco said.
The head of the hospital where three people were still listed in serious condition told the G1 news site that no one had lost their hands, as originally reported.
The UN statement urged a "rapid and impartial" investigation into the episode and for authorities to offer protection to victims and witnesses if necessary.
Indigenous organization accuse President Michel Temer's conservative government of handing off indigenous rights issues to public servants linked to a powerful faction in Congress that backs ranchers' and farmers' interests.
The Gamela tribe asserts that the disputed land was donated to them in Portuguese colonial times, but that they were forced off it from the 1970s by encroaching ranchers.
From 2015, indigenous groups have been occupying land they claim, sparking confrontations with ranchers and farmers.
According to a church-linked group, the Pastoral Commission of the Earth, 61 people were murdered in Brazil last year in rural conflicts, most of them landless or occupying country dwellers under pressure from big ranchers.
Brazil's justice minister, Osmar Serraglio, has promised to get processes under way to clearly designate the boundaries of protected indigenous land.
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