A senior army officer and a motorcyclist and two members of an armed group have been killed in separate incidents in Burundi, officials said on Thursday, a day after the latest flareup of violence in the country's year-long crisis.

Tit-for-tat killings by supporters of President Pierre Nkurunziza and his political opponents have raised concerns that Burundi could slide back into conflict, after the country emerged from an ethnically fuelled civil war in 2005.

More than 400 people have been killed since Nkurunziza said he would run for a third term last year. Opponents said he was violating the constitution and a peace deal that ended the civil war. A court ruled he could run, and he won re-election in July.

In one incident on Wednesday, gunmen ambushed Colonel Emmanuel Buzubona as he travelled on the back of a motorbike to his home in Bujumbura, the capital, according to Moise Nkurunziza, deputy police spokesman.

"The senior officer tried to flee but he didn’t make it because the attackers finished him off by throwing a grenade," Nkurunziza told Reutershe said. The motorcyclist also died.

The motive was not immediately clear.

Buzubona's neighbours said he had returned in September from Tanzania, where he had been an instructor at a military college for officers from the East African Community bloc. He was awaiting a new assignment, army spokesman Gaspard Baratuza said.

An ethnic Hutu, Buzubona had been a member of CNDD-FDD rebel force, which became the ruling party of the president.

In the civil war, Hutu rebel groups such as CNDD-FDD fought the then-Tutsi-led army. The army has since been reformed to include both Hutus and Tutsis in all ranks. Leaders of a failed coup in May included both Hutus and Tutsis.

In a separate incident on Wednesday, soldiers on patrol killed two people in a clash with an armed group in Kivumu village in Mugamba southern district, 60km southeast of Bujumbura, said Christian Nkurunziza, regional governor of the district where Mugamba is located.

"As they were patrolling, soldiers exchanged fire with an armed group of eight men, killing two people," he said.

Four members of the CNDD-FDD party were killed in the same village last week.

About 250,000 people have fled since violence erupted, most to border camps in neighbouring Tanzania.