Burundian security forces attempted to cover up state-sponsored killings in December by throwing dozens of bodies in mass graves, according to a report from Amnesty International released on Thursday. The revelations come as the African Union begins its annual summit with the ongoing crisis in Burundi looming.

Information gathered through witness testimony, satellite images, and video footage present indicates that dozens were killed by security forces in the East African country following a rebel-led attack on December 11 waged against several military bases in the capital Bujumbura, Amnesty alleges. In the aftermath in the days that followed, the authorities reportedly collected the bodies and buried them in mass graves.

Reports quickly began to circulate about the existence of mass graves following the December violence, which began early in the morning that Friday and resulted in firefight between the attackers and security forces that lasted late into the night. Through an investigation, Amnesty determined that bodies had not turned up at the city's four main morgues after the violence as expected. Later the organization reviewed satellite imagery that indicated graves likely existed in an abandoned ceremony near Burundi's international airport, along with several other potential locations on the outskirts of Bujumbura.

With the disappeared bodies, it has been challenging for Amnesty and other human rights groups to come up with a more accurate death toll or determine how people were killed. Muthoni Wanyeki, Amnesty International's regional director for East Africa, the Horn and the Great Lakes, called for the grave sites to be secured until investigations can get underway.

"These images suggest a deliberate effort by the authorities to cover up the extent of the killings by their security forces and to prevent the full truth from coming out," Muthoni Wanyeki said. "Families need to know what happened to their loved ones and to be able to bury them in dignity."

Hundreds have died in Burundi since President Pierre Nkurunziza's decision to seek a controversial and constitutionally questionable third term in office sparked unrest in April 2015. Young protesters and opposition members took to the streets on the heels of the spring announcement to demonstrate against the president's effort, with the rallies quickly turning violent as police officers waged a brutal crackdown and opposition and government supporters clashed.

Nkurunziza, a 51-year-old former rebel leader who became president after the country's decade-long civil war came to a close in 2005, was re-elected in a 2010 election that also saw outbreaks of political violence. The constitution, established in line with the Arusha peace agreement outlined after the war, sets out a strict two-term limit for president. Nkurunziza's supporters, however, argued that he was still eligible because he had been appointed to his first term by parliament rather than directly elected. The constitutional court ultimately sided with the president and he won re-election in July, with the situation subsequently shifting to politically motivated violence, disappearances, and assassinations on both sides. Some 240,000 people have fled to neighboring countries like Rwanda and Tanzania.

The situation escalated quickly at the end of the year and fears began to arise that Burundi was on the brink of war, particularly in the aftermath of the December 11 violence. After the attacks, the United Nations High Commissioner for Human Rights Zeid Ra'ad al-Hussein specifically warned that the landlocked country was on the cusp of civil war and called for a robust and decisive global response.

Later that month the African Union (AU) drew up a proposal to send 5,000 troops to Burundi, leaving open the possibility to invoke a rule that has never been used and would allow the body to deploy into a country without its consent. Burundi said there was no need for a peacekeeping mission in its borders and has maintained that position.

This week, however, as the AU meets for its 26th annual session in the Ethiopian capital of Addis Ababa, the issue of sending peacekeepers to Burundi was said to be a priority on the sidelines. The emphasis on accepting peacekeepers will come on the heels of the UN Security Council's visit to Burundi last week, in which members failed to make any progress in convincing Nkurunziza on the peacekeeping issue.

Senior AU officials told Reuters that leaders would be urging Nkurunziza to allow the peacekeeping plans to move forward. Considering the president's previous comments against the situation, however, the official was reportedly not optimistic that Nkurunziza would change his mind.

"He is not expected to endorse the plan, however," the diplomat said, adding that they could impose sanctions in the event that he refuses.

As the AU session gets underway, groups like the International Federation for Human Rights have called on the AU to shore up a response.