BUJUMBURA, Burundi -- The Canadian and U.S. governments asked their citizens to leave Burundi as soon as possible Sunday after 87 people were killed Friday in an escalation of the violence surrounding the disputed third term of President Pierre Nkurunziza.

Right activists say the Dec. 11 fighting was the worst in months of turmoil since April, when Nkurunziza's candidacy was announced, and accused the security forces of extrajudicial killings of civilians.

Foreign Affairs Minister Stephane Dion issued a statement Sunday saying that Canadians should leave while commercial flights are still available.

"Canada is deeply concerned by the escalating violence and incidents of brutality reported in Bujumbura (Burundi's capital) in recent days. We are also alarmed by the high number of victims killed under circumstances that remain unclear. These violent acts and the ongoing repression that continues to violate human rights must stop," the statement said.

Due to the continuing violence the U.S. State Department announced Saturday that it will pull out all non-emergency U.S. government personnel and dependents of U.S. government employees. The U.S. Embassy can only offer limited emergency services to U.S. citizens in Burundi, the State Department said in a travel warning.

Political violence persists throughout Burundi following the country's contested elections, an attempted coup and the controversy over Nkurunziza standing for and winning a third term, the warning said.

Gunfire and grenade attacks occur with frequency from armed groups operating in Burundi, but are usually not directed at foreigners, it said.

"Demonstrations, gatherings, and even sporting events that are intended to be peaceful can turn violent without advance warning," it said.

Last month, Belgium, Burundi's former colonial power, advised its citizens to leave the Central African country, amid rounds of killings of opposition and government supporters.

On Friday armed men attacked three army installations. More than 150 men raided the army facilities and 79 of them were killed, army spokesman Col. Gaspard Baratuza said Saturday. Eight security agents, four from the army and four from police, also died in the fighting and 21 security officers were wounded, he said. Baratuza said forces arrested 45 members of the unidentified group that carried out the attacks.

After the attacks, armed forces Friday night went door-to-door in some neighbourhoods and dragged people from their homes and shot them dead, said residents. At least 28 bodies were found in the streets, many shot in the head and with their hands tied behind their backs.

A precarious calm settled in Bujumbura Sunday as businesses opened and movement across the capital resumed.

"This is by the far the most serious incident, with the highest number of victims, since the start of the crisis in April," said Carina Tertsakian, a senior researcher at Human Rights Watch, of Friday's violence.

The army's killings of people in residential neighbourhoods appears unjustified and the security forces responsible should be held to account, she said. A serious investigation should be carried out, with the help of experts from outside the country, given the politicization and corruption in the Burundian justice system, said Tertsakian.

Burundi could be slipping back to civil war, warned Anschaire Nikoyagize, the president of the Burundian League for Human Rights. He accused security forces of targeting people of different political views and everybody who opposed Nkurunziza's third term.

More than 300 people have been killed and about 215,000 others have fled Burundi since April, when it was announced that Nkurunziza would stand for a third term, which he won in elections in July.

Many Burundians and the international community have opposed Nkurunziza's third term as unconstitutional and in violation of a peace accord. The treaty ended a civil war in which 300,000 people were killed between 1993 and 2006.

Associated Press Writer Ignatius Ssuuna contributed to this report from Kigali, Rwanda.