The meeting will bring together President Pierre Nkurunziza’s government and opposition representatives in Arusha from November 27 to December 8.
Envoy Michel Kafando told the UN Security Council the CNARED opposition alliance “was not ready to take part.”
East African leaders will then meet in January to assess the outcome of the talks in Arusha, he said.
Burundi was thrown into a political crisis in April 2015 when Nkurunziza decided to run for a third term that his opponents said was unconstitutional.
Since then, between 500 and 2,000 have been killed, according to various sources, while over 400,000 have fled the country.
Last month, the CNARED reacted with dismay when the government approved changes to the constitution that would pave the way for 14 more years of power for Nkurunziza.
CNARED president Charles Nditije said Nkurunziza had crossed a “red line” and “definitively buried” the Arusha peace accord that sets a limit of two five-year presidential terms.
Reached in 2000, the peace accord ended a brutal civil war that left 300,000 dead.
In power since 2005, Nkurunziza was selected by parliament and went on to win elections in 2010 and was re-elected in 2015.
“The Burundian authorities are already preparing the next elections even if there has not yet been a fair solution to the issue of an inclusive dialogue,” said Kafando.