The United Nations has offered to send independent forensics experts to Burundi to help authorities investigate allegations of mass graves that are said to have been discovered in different areas of the country.

After a government security crackdown in December, witnesses came forward with accounts of at least nine mass graves in and around Bujumbura including one in a military camp where more than 100 bodies were allegedly buried.

Corpses were allegedly dumped in grave sites by police who had carried out raids in Bujumbura, arresting and shooting young men in response to a December 11 attack by gunmen on three military barracks.

UN Assistant Secretary-General for Human Rights Ivan Simonovic told reporters that a Burundian prosecutor had opened an investigation of the alleged mass graves and that the United Nations had offered to help.

"Somebody has to dig those mass graves. We would like to be present when this is done and we are offering to provide forensics experts," Simonovic said.

The government, which has dismissed the allegations, has not responded to the UN offer for experts to be on site for the excavations.

UN human rights officials are examining satellite imagery of the alleged mass graves and pressing the government to act quickly to shed light on the allegations.

Burundi has been in turmoil since President Pierre Nkurunziza announced plans in April to run for a third term, which he went on to win.

More than 400 people have died in the violence and at least 230,000 have fled the country.

UN Security Council ambassadors travelled to Burundi last month to urge Nkurunziza to open up serious negotiations with the opposition and agree to an international presence.

The mission was aimed at preventing a slide toward ethnic killings in Burundi between Hutus and Tutsis, similar to the violence that led to the 1994 Genocide against the Tutsi.

Agencies