Burundi's government will not take part in peace talks with the opposition scheduled for Wednesday because it holds some of the participants responsible for recent months of violence, a senior official said.
The talks in neighbouring Tanzania were announced last month as part of regional efforts to resolve a crisis triggered by President Pierre Nkurunziza's decision to run for a third term in office - a move opponents said broke the constitution.
International monitors say they fear Burundi could be headed for civil war.
The government refused to join the talks due to the inclusion of "those who are supporting violence", Joseph Bangurambona, the permanent secretary in Burundi's foreign affairs ministry, said.
"No dialogue tomorrow, neither on January 16 as many may think, because there has been no consensus on that date," Bangurambona told Reuters on Tuesday.
He said the government was particularly opposed to the inclusion of Carine Kaneza, representing Burundian women, adding the government did not recognise her role or the organisation she represents.
He said Kaneza was fired from her job as secretary at the Burundian embassy to the United States last October due to "her mistakes". Kaneza was not immediately available to comment.
"The dialogue will resume on condition that the mediators iron out the irregularities and set another agreed-upon date," Bangurambona said.
Uganda, whose President Yoweri Museveni is mediating in the Burundi crisis, confirmed: "The Burundi peace negotiations will not resume tomorrow and at this moment we can't tell when they will."
Paddy Ankunda, spokesman for the Ugandan military, added: "What is going on is that all the parties involved are still making consultations; talks of this nature are never easy."
Thacien Sibomana, the spokesman for the opposition UPRONA party, said they had not received an invitation for the talks on Wednesday, and accused the government of railroading.
"This is a government strategy to drag on this dialogue while people are continuously being killed. That is why we ask an urgent deployment of troops," Sibomana said.
Two people were injured in a grenade attack in the capital Bujumbura on Monday, the latest in the conflict. Police said they were investigating.
The United Nations estimates 400 people have already been killed. More than 220,000 people have fled Burundi since April, and Rwanda, which has similar ethnic faultlines, says more than 73,000 Burundians are now on its soil.