The announcement came a day after the 54-member bloc said it would send a 5,000-strong forced to halt spiralling violence in the tiny central African country as fears grow it is rapidly sliding towards civil war.
It gave the government in Bujumbura a four-day deadline to agree to the offer, but warned it would send troops anyway.
“Burundi is clear on the matter: it is not ready to accept an AU force on its territory,” deputy presidential spokesman Jean-Claude Karerwa told AFP.
“If AU troops came without the government’s approval, it would be an invasion and occupation force, and the Burundi government would reserve the right to act accordingly.”
Burundi has so far dismissed proposals for any peacekeeping force on its territory and Karerwa said any such move by the AU would have to be approved by the United Nations Security Council.
“The Burundi government believes the AU resolution cannot be automatically applied and must first be endorsed by the UN Security Council,” he said.
The standoff comes as international alarm grows over soaring unrest in Burundi where at least 87 people were killed on December 11 in a crackdown by security forces after an attack on three military bases.
Many of the dead were youths who were shot dead by the security forces.
In a strongly-worded statement issued on Saturday, the AU said the bloc would “take additional measures” to ensure the new force’s deployment.
It underlined its determination “to take all appropriate measures against any party or actor… who would impede the implementation of the present decision”.
The announcement came two days after the bloc’s Peace and Security Council met over the Burundi crisis and agreed it would not allow “another genocide” on African soil.
AU rights investigators last week returned from a fact-finding mission to Burundi expressing “great concern” after witnessing some of the heaviest fighting in the troubled country for months.
The AU team said it had reports of “arbitrary killings and targeted assassinations” as well as arrests, detentions and torture. Their concerns have been widely echoed.