Burundi's spy agency has executed at least 38 people and detained 200 others since the new government came to power, a human rights group claims.
The New York-based Human Rights Watch said the year-old government had failed to prosecute those accused of the extra-judicial killings.
"Since the new government took office, the intelligence service has been free to use any means necessary, including killing and torture, to reach its goals," said Alison des Forges, senior Africa adviser at Human Rights Watch. "The government must address this pattern of continuing violations."
A Burundian security officer said the cases were isolated and that those responsible were punished.
The rights group said only two suspects had been arrested for the killings of 34 people but, as yet, no one has been charged. The group said it had evidence to show the spy agency was responsible for the crimes.
The new, Hutu-majority government was elected in an atmosphere of optimism in a country recovering from a civil war that killed more than 250,000 people.
Burundi has long been riven by tension between the majority Hutus and minority Tutsis, who have dominated the government since independence from Belgium in 1962. Hutu-Tutsi animosities also brought bloodshed in neighbouring Rwanda - where, in 1994, militant Hutus killed an estimated 800,000 Tutsis and moderate Hutus.