The government of Burundi has said it will not attend peace talks scheduled to resume in Tanzania on Thursday, although the main opposition alliance has confirmed its participation.
The talks are meant to find an end to a violent political crisis that began in 2015 after President Pierre Nkurunziza said he would seek a third term - a move opponents said violated the constitution and a peace deal that ended an ethnically charged civil war.
"The government of Burundi finds some irregularities in the organisation of this present session," a government statement said on Wednesday.
Spokesman Phillipe Nzobonariba said the government objected to the presence of senior UN adviser Benomar Jamal, but did not say why.
The government has repeatedly accused the UN of bias against it after several human rights groups have said the security forces and ruling party have committed abuses.
The main opposition grouping, CNARED, said it would attend the talks although it has previously accused mediator Benjamin William Mkapa, a former president of Tanzania, of bias.
Mkapa said in December that Nkurunziza was legitimate and that mediators should focus on setting up elections for 2020.
The violence in Burundi has alarmed people in a region where memories of the 1994 genocide in neighbouring Rwanda remain raw.