Burundi said it would refuse to allow United Nations police onto its territory to monitor the security and human rights situation after the UN Security Council voted to send 228 officers.
More than 450 people have been killed since President Pierre Nkurunziza won a third term last year, a move his opponents say violated the constitution and a peace deal that ended a civil war in 2005. Government and opposition officials were among those killed in tit-for-tat violence by rival sides.
About a quarter of a million people have fled the violence, which has alarmed neighbouring countries in a region where memories of Rwanda's 1994 genocide remain raw. Like Rwanda, Burundi has an ethnic Hutu majority and a Tutsi minority.
The Security Council voted on Friday to authorise the police deployment, though four of the 15 members abstained.
"Concerning the deployment of the police force, the government of Burundi reminds the Security Council that ... every resolution ... has to be approved by the host country, which was not, unfortunately, the case," government spokesman Phillipe Nzobonariba said in a statement late on Tuesday.
"The government ... rejects any resolution measure in connection with sending any force on its territory in violation of elementary rules governing the family of United Nations and especially violating the sovereignty of its territory."
Burundi's UN Ambassador Albert Shingiro said in July his country would only accept up to 50 unarmed UN police.
The United Nations needs approval from Burundi's government to send the police.