The New York-based group Human Rights Watch has accused Vietnam of repressing a minority Christian community in the Central Highlands.
Minority groups have several grievances against the government
The group said dozens of Montagnards were missing after security forces used electric truncheons, tear gas and water cannon to disperse a weekend protest.
Human Rights Watch said that some protesters were beaten to death, while many others were still missing.
The Vietnamese government has denied the allegations.
The claims cannot be independently confirmed, as the Central Highlands area has been sealed off to all foreigners, including journalists.
Human rights at 'new low'
A large crowd of Montagnard tribes-people demonstrated in front of local authority offices in Daklak province on Saturday, protesting
against religious repression and confiscation of their tribal lands.
It is not the first time the Montagnards have held demonstrations on this issue. An outbreak of protests over land rights and repression led to a government crackdown in 2001.
But Brad Adams, executive director of the Asia division of Human
Rights Watch, said on Wednesday: "The human rights situation for Montagnards in the Central Highlands has plummeted to a new low."
Religion in Vietnam is controlled by the state
"Vietnam's policy of repression of Montagnard Christians is only fuelling the unrest," he said.
Foreign Ministry spokesman Le Dung denied the claims. He said in a statement: "Functional organs in Daklak said there is absolutely nobody who was beaten to death as (stated in) the news by Human Rights Watch. We strongly reject this information."
Western diplomats, including from the US embassy, have
met officials at the Vietnamese foreign ministry to register their concerns.
Many Montagnards are Protestants who follow an Evangelical Christian Church not sanctioned by the government.
Correspondents say the communist government in Vietnam officially guarantees religious freedoms, but both majority Buddhists and minority Christians have complained of violations to their right to worship.