Burundi police said on Saturday they had detained a man they had identified as a Rwandan soldier and alleged that he was in the country on a mission to "destabilize" Burundi.
The incident comes amid heightened tensions between the neighbouring countries that have similar histories of ethnic violence.
Burundi has been in turmoil since last April, when President Pierre Nkurunziza’s decision to seek a third term sparked weeks of street protests. Rights groups have warned the country could be slipping towards civil war.
A confidential report to the United Nations Security Council earlier this year accused Rwanda of recruiting and training Burundian refugees with the goal of ousting Nkurunziza.
The Rwandan man was arrested close to the border with Tanzania, about 250 km from the Burundian capital of Bujumbura, said police spokesman Pierre Nkurikiye.
"The aim is to destabilise our country ... It’s another proof of destabilisation of our country by our neighbour Rwanda," he said.
In an address earlier on Saturday Rwandan President Paul Kagame said that his country had no involvement in Burundi.
Burundi and Rwanda have the same ethnic mix, about 85 percent Hutus and 15 percent Tutsis. A 12-year civil war in Burundi, which ended in 2005, pitted a Tutsi-led army against Hutu rebel groups. (Additional reporting by Clement Uwiringiyimana; Editing by Edith Honan and Greg Mahlich)
By Robert Muriisa.