BUJUMBURA, Burundi — Gunfire rang out in Burundi's capital Sunday night following the killing of a feared military general who was a close ally of President Pierre Nkurunziza.

Nkurunziza, who was re-elected last month, said in a televised address that he has ordered the police to find Lt. Gen. Adolphe Nshimirimana's killers within 10 days.

The killing of Nshimirimana — who was the senior presidential adviser for internal security — could spark revenge killings and further fuel violence that stemmed from Nkurunziza's controversial bid for a third term. Nshimirimana was killed in a drive-by shooting early Sunday in the capital, Bujumbura.

Carina Tertsakian, who researches Burundi for Human Rights Watch, said Nshimirimana was "one of the key hardliners around the president" and became even more influential as Nkurunziza faced regular street protests by civilians who wanted Nkurunziza to retire after serving for two terms.

"Despite or perhaps because of his brutal reputation, Adolphe was generally seen as untouchable, with no one in a position of power daring, or even suggesting, holding him to account," she said.

Human Rights Watch has received frequent allegations that he was behind many incidents of killing, torture, arrests of suspected opponents and other abuses over the past several years, she said.

Nshimirimana, a former army chief of staff as well as head of the intelligence services, is believed to have helped to defeat an attempted coup against Nkurunziza in May.

The U.S. has condemned the killing and is urging restraint.

Nkurunziza won the July 21 presidential polls with 69 percent of the vote. The international community condemned the elections as not credible because of violence, intimidation, media restrictions and questions over the legitimacy of a third term for Nkurunziza.

Associated Press reporter Rodney Muhumuza in Kampala, Uganda, contributed to this report.