DAKAR, Senegal (AP) — Two more African Union peacekeepers were killed overnight in the Central African Republic, officials said Friday, bringing to 11 the number of international peacekeepers killed in less than a month and underscoring the dangerous nature of the unfolding conflict in the nation.

The two officers from the Republic of Congo were killed by unidentified assailants, African Union mission spokesman Eloi Yao told The Associated Press by telephone from Central African Republic. Their deaths come a day after a pickup truck carrying Chadian peacekeepers was attacked with heavy arms fire, incinerating the vehicle and killing six of the soldiers inside. Fifteen others were wounded in that attack. Earlier this month, two French soldiers were also killed in Bangui, the country's troubled capital.

"Last night, two police officers with the MISCA (the African Union peacekeeping force) from the Republic of Congo were killed by unidentified gunmen after they ambushed them during a patrol in the town of Bangui," said Yao.

Calm appeared to have returned to Bangui on Friday, after heavy fighting which came perilously close to the presidential palace, according to presidential spokesman Guy Simplice.

Also on Friday, the country's attorney general Ghislain Gresenguet announced that he had opened an investigation into a mass grave discovered not far from the palace, on a hillside. At least 20 decomposing bodies were found, he said. The remains were several days old and Gresenguet said the wounds they bore indicated the victims had likely been tortured before their death.

"Some of the bodies were bound, their hands tied together with rope. Other bodies were mutilated, with large wounds. Though we don't know if they were caused by firearms or by machetes," said the chief prosecutor by telephone.

Earlier this month, the United Nations Security Council authorized a French and an African Union intervention mission. A total of 1,600 French troops and some 3,000 African troops have been deployed since Dec. 5 to the country which some officials have described as on the verge of genocide.

Central African Republic began its slide into anarchy nine months ago, following a coup by a Muslim rebel group. They stormed the capital, and forced out the nation's Christian president, as well as the mostly-Christian elite presidential guard. The soldiers loyal to the former regime are believed to be backing a Christian militia, which has led repeated attacks on the capital, as well as targeted killings of Muslims, and assaults on mosques. The level of violence has shocked even hardened Africa watchers, with Christian youth in the capital openly lynching Muslims, who are accused of complicity with the rebels who seized power.

An Associated Press journalist saw young men parading in the streets with the severed penis of one of their victims, and with the hacked-off foot of another.

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Associated Press writers Rukmini Callimachi in Dakar, Senegal and Rebecca Blackwell in Bangui, Central African Republic contributed to this report.