His threats come after the AU revealed its plans to send about 5,000 troops to the East African nation, without the permission of the government.
The BBC reports that “at least 400 people have been killed and 220,000 displaced since April in Burundi” a situation which drove the AU to make their decision.
However, the Burundian president has warned that his country is prepared to fight if their borders are breached by the AU troops without their consent.
“Everyone has to respect Burundi borders,” Mr Nkurunziza said.
He added that it would be seen as “an attack on the country and every Burundian will stand up and fight against them,” if the AU forces enter Burundi.
Under Burundi’s constitution, foreign troops can only intervene if the warring parties ask for it, or if there is no legitimate government in place, the president said in comments broadcast on state radio.
President Nkurunziza’s announcement that he would seek a third term in office sparked violent clashes in the country with several experts fearing that it could escalate to civil war.
His disputed landslide victory in July’s elections after he survived a coup attempt two months earlier, hasve not done much to endear him to the opposition who feel it’s time for him to step down from office.