At least four people were killed in fresh grenade and gun attacks in Burundi ahead of a visit Monday by UN chief Ban Ki-moon aimed at ending months of deadly unrest.

Ban is due to arrive in the capital Bujumbura from New York at 4:30pm (1430 GMT) for talks with Burundian President Pierre Nkurunziza on Tuesday, after meeting with leaders of political parties and civil society.

"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 said.

"After the grenade thrown in the Kwa Siyoni market in the Ngagara district that left one dead and seven hurt, a second grenade exploded later in Kamenge market, leaving one person dead and two hurt," Bujumbura mayor Freddy Mbonimpa told AFP.

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

Witnesses and the mayor said the assailants on motorbikes wrapped the grenades in plastic bags before throwing them.

Mbonimpa said three men had been arrested in connection with the violence.

"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," he added in reference to Ban's brief 24-hour visit.

Bujumbura has been hit by a wave of grenade attacks by assailants on motorbikes, leading the mayor last week to slap a ban on the use of the popular mode of transport in the city.

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.

Nyakarerwa said it was the fourth such attack in the commune in the last two weeks.

Authorities have used the term "armed criminals" to describe many of the rebels attempting to drive Nkurunziza out of office by violent means.

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.

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.