Fishermen report dozens of corpses carried down the Akagera river, which has overtones of previous political violence

Mystery surrounds the discovery of dozens of bodies wrapped in plastic and dumped in a lake bordering Burundi and Rwanda.

Fishermen in Burundi claim to have found 40 corpses floating in Lake Rweru since July, some with their limbs bound. A government spokesman said this figure is probably exaggerated but an investigation is under way.

There is no clue about the motive for the killings or the identity of the perpetrators or victims. Authorities in Burundi and Rwanda said their citizens are not among the deceased.

Edouard Nduwimana, Burundi's interior minister, told Agence France-Presse: "There are many bodies, some tied, floating on Lake Rweru, although probably the figure of 40 reported by fishermen is exaggerated. But it is certain that there are many floating bodies in the lake."

Five have been found in the past week, according to local government officials. On Monday a joint Burundi-Rwanda government and police team discovered three bodies, including two wrapped in a big bag.

Nduwimana said that while the fisherman reported the bodies had been carried into the lake from the Akagera river, downstream of Rwanda, it was not yet possible to confirm their origin. "Investigations are needed to confirm whether or not these bodies come from Rwanda, or whether they were thrown into the lake from Burundi," he said.

Regional authorities said their investigations suggest that the victims were not from Burundi.

On Tuesday Rwandan police denied that they were Rwandan citizens either. Theos Badege, the commissioner for the criminal investigation department, said: "We can confirm that there are no missing Rwandans among the bodies." Local mayors in Rwanda also said no citizens in their area have been reported missing.

Rwandan president Paul Kagame has been accused of human rights violations including the assassination or disappearance of opponents. Political tensions are rising in Burundi before elections next year.

During the Rwandan genocide in 1994, bodies of some of those killed floated down the Akagera river. In recent years bodies of Burundian opponents murdered in political violence have also been thrown into various rivers in the country.