In coordinated attacks, gunmen stormed three military installations in Burundi on Friday. At least 15 people were killed as gunfire and explosions rocked the capital, marking a steep escalation of a simmering conflict.

Around 4 a.m., the unidentified attackers wearing civilian clothing hit two military installations in the capital and one in the countryside, showing a level of military sophistication. Terrified civilians in Bujumbura, the capital, stayed in their homes as stray rounds hit some of them.

The sounds of battle continued into the afternoon Friday with military and police vehicles the only ones on the deserted streets. Roadblocks were set up in part of the city, residents said.

"A stray bullet hit the wall of my neighbor's house. We do not know what going on in the streets. We are living fear," said Claire Biguda, a resident of Nyakabiga neighborhood, who was locked up in her house along with her husband and two children.

The International Committee of the Red Cross tweeted: "We are unfortunately unable to move around town for now, hoping to be allowed to provide assistance quickly."

Burundian officials have previously accused neighboring Rwanda of supporting an insurgency against President Pierre Nkurunziza. It was unclear if these were the attackers. There was no immediate comment from Rwanda.

In July, Burundi's army said it has captured more than 200 armed men who formed part of the insurgency. When interviewed by the press, some of the men confirmed they have been trained in Rwanda, without giving details. Recently, Burundi's prosecutor general wrote a letter to the justice officials in Rwanda, seeking the extradition of Burundian generals who had fled this country after a failed coup of last May.

Twenty of the attackers were arrested, including one who was wounded and is being treated at a military hospital, military spokesman Col. Gaspard Baratuza told state radio.

The attackers wanted to steal weapons and use them free prisoners, Baratuza said. Several hundred people have been imprisoned for opposing Nkurunziza's election this year to a third term.

Baratuza said five soldiers were wounded in the attacks. However, military officials, who insisted on anonymity because they were not authorized to speak to the media, said three soldiers were killed.

Five of the attackers and two soldiers were killed in the assault at a camp in Ngagara neighborhood, a soldier said. Another soldier at the ISCAM military academy said one soldier died there.

A third attack took place in Mujejuru in the commune of Mugongo Manga, around 35 kilometers (22 miles) from the capital, Baratuza said.

Friday's fighting is apparently part of violence linked to Nkurunziza having been elected to a third term, which many Burundians and foreign observers had opposed as unconstitutional and in violation of peace accords that ended a civil war in which 300,000 people were killed between 1993 and 2006.

Nkurunziza, who took power in 2005, won elections in July for a third term. The Constitutional Court ruled in favor of Nkurunziza, who says he was entitled to another term because for his first term he was elected by parliament and not by popular mandate. The deputy president of the Constitutional Court fled to exile in Rwanda before the ruling and said the court had been coerced to rule in favor of the president.

At least 240 people have been killed since April and about 215,000 others have fled to neighboring countries, according to the U.N.

At least seven people were killed this week, six by men wearing police uniforms.

Burundi has a history of deadly conflicts between the country's Hutu and Tutsi ethnic groups, though the current violence appears more politically than ethnically motivated.

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This version corrects that it was the Constitutional Court that ruled in favor of Nkurunziza, not the Supreme Court.