The UN has warned that Burundi risks sliding back into civil war due to a dramatic upsurge in killings following the controversial re-election of President Pierre Nkurunziza.

Zeid Ra'ad Al Hussein, UN High Commissioner for Human Rights, said in a statement on Monday that many of the victims had reportedly been detained by Burundi's National Intelligence Agency (SNR) before their deaths.

"Almost every day, dead bodies are found lying on the streets of some of [the capital city of] Bujumbura's neighborhoods," Al Hussein said, adding, "In many cases, the victims appear to have been killed by a bullet fired at close range. The bodies sometimes show signs of torture and are typically found with their hands tied behind their backs."

"This succession of unexplained killings, and the widespread perception that they may be linked to state institutions, is instilling a deep sense of fear within the population, especially in neighborhoods known to be supportive of the opposition," he said.

Meanwhile, Zeid's office has said that it had registered over 130 killings, over 90 cases of torture and hundreds of cases of arbitrary detention since April.

Over 700 people have already been arrested by the Burundian government forces this month alone, it added.

Zeid also warned that "more people are looking to take the law into their own hands" amid an ongoing tit-for-tat culture of violence. "There is an increasing risk that spiraling tit-for-tat violence will plunge the country back into its bloody past."

The UN rights chief also said the ongoing security operations across the country are seriously violating the residents’ right to free movement.

Zeid also touched on the government's inaction over the killings, saying, "We have also been receiving many allegations of torture carried out by the police or the SNR, with the reported aim of forcing victims to confess to participation in an armed rebellion."

"The perpetrators of both torture and killings appear, to date, to have enjoyed absolute impunity," said Zeid.

Burundi's government has not yet commented on this report.

An impoverished and landlocked country in the heart of the troubled Great Lakes region of Central Africa, Burundi plunged into turmoil in late April, when Nkurunziza first announced his bid to run for a third consecutive five-year term.

Nkurunziza won an outright victory in the presidential election after grabbing 69.41 percent of the votes. His third term has widely been censured as unconstitutional by the country’s opposition.

The decision was denounced by the opposition, which argued the move was contrary to the constitution, which only allows two successive terms, and the 2000 Arusha Agreement that paved the way for ending the civil war in the country.

Burundi had already been struggling to emerge from a 12-year, ethnic-based civil war lasting from 1993 to 2005, leaving around 300,000 people killed. The country has been plagued by tension between the usually-dominant Tutsi minority and the Hutu majority since independence in 1962.