Burundi's last remaining rebel group, the National Liberation Forces (FNL), has rejected an offer of peace talks with the new government.
President Nkurunziza hopes to bring his rivals into the peace process
The government took office last month under a UN-backed peace process aimed at ending a war between Hutu rebels and an army led by the Tutsi minority.
The FNL was the only rebel group to remain outside the peace process.
A five-year interim constitution guarantees a balance of power between Burundi's Hutu and Tutsi people.
"We ignore the current government led by Pierre Nkurunziza, because this government was imposed by the international community and it was not elected by Burundian people," FNL spokesman Pasteur Habimana told the Reuters news agency.
On the eve of his inauguration last month, President Nkurunziza vowed to engage the FNL in peace talks.
Some 300,000 people were killed in the civil war, which was sparked in 1993 by the assassination of Burundi's first Hutu head of state and democratically elected president, Melchior Ndadaye.
Mr Nkurunziza is the leader of the Forces for the Defence of Democracy (FDD), which was previously the main rebel group fighting a Tutsi elite which has dominated Burundi since independence.
The FDD joined the peace process and entered the transitional power-sharing government in 2003.
Mr Nkurunziza became president in August after the FDD swept elections earlier this year.
Peace in Burundi is seen as essential to a wider settlement in the troubled Great Lakes region, but analysts say that the country's peace prospects depend on Mr Nkurunziza's success in bringing the rival FNL group into government.