At least four people were killed in fresh attacks ahead of UN Secretary General Ban Ki-moon's arrival in Bujumbura Monday, his first visit to Burundi since a crisis began in April last year.

Ban, who landed late on Monday afternoon, is due to hold talks with President Pierre Nkurunziza on Tuesday after meeting with leaders of political parties and civil society aimed at giving fresh impetus to stalled efforts at resolving the 10-month-old crisis.

Ban is expected to leave Burundi after the talks and visit the Democratic Republic of Congo on the second leg of an Africa tour that will also take him to South Sudan where civil war erupted in December 2013.

"This is a very important visit because we hope the UN Secretary-General will bring his weight to bear on President Pierre Nkurunziza so he finally accepts an inclusive and unconditional dialogue with his opponents," a UN official told AFP.

Hours before the UN chief's arrival at least two people were killed and nine hurt in two separate grenade attacks in the capital early Monday by assailants on motorbikes, the latest in a string of such attacks, the city's mayor Freddy Mbonimpa said.

Mbonimpa denounced the attacks as a "terrorist act targeting peaceful citizens".

"The capital had been calm for several days, but these terrorists as usual launch their criminal operations to show they're here just when Burundi is expecting an important guest," the mayor added in reference to Ban's brief 24-hour visit.

It remains unclear who carries out the grenade blasts, which have increased in the past few weeks. Security forces, rebels and opposition all blame each other for the killings.

In a separate incident, two people were killed and two injured Sunday night in the commune of Gisozi southeast of Bujumbura in an attack by "armed criminals", local governor Jean-Marie Nyakarerwa told AFP. 

The violence comes as Ban hopes to revive efforts aimed at finding a peaceful solution to the political crisis.

A Western diplomat in Burundi said he hoped Ban would also discuss, "massive violations of human rights observed since the beginning of the crisis."

"We hope the visit will convince (Nkurunziza) to accept a genuine international investigation into these allegations," the diplomat said.

Burundi was thrown into crisis last April when Nkurunziza decided to run for a controversial third term that he went on to win in July. Since then, clashes between loyalists and the opposition have turned increasingly violent.

More than 400 people have been killed and over 240,000 have left the country since then, while thousands more have been arrested and the security forces repeatedly accused of extrajudicial executions and other human rights violations.

Ban's visit comes as the Burundi government appeared to soften its position towards its opponents agreeing to receive a delegation of African heads of state expected later this week and cancelling international arrest warrants against several exiled opposition leaders.

The UN has warned Burundi risks a repeat of the 1993-2005 civil war, with hundreds dead since April and more than 230,000 people fleeing to neighbouring countries.