BUJUMBURA, Sept 13 (Reuters) - Burundi has opened an investigation into claims that a former ethnic Hutu rebel leader ordered the massacre of Congolese refugees in an attack almost a decade ago, its prosecutor's office said on Friday.
The Banyamulenge, an ethnic Tutsi tribe from neighbouring Democratic Republic of Congo, in August accused Agathon Rwasa of ordering the killing of 166 Banyamulenge seeking refuge inside a camp in Burundi in 2004.
Rwasa, who led the FNL (National Liberation Forces) during Burundi's civil war, denies involvement in the slaughter and said the probe was politically motivated, aimed at hampering his bid to run for the presidency for a second time in 2015.
"This is a way of keeping me out of the political arena so that I can't contest the next elections," Rwasa told journalists at his home in a wealthy suburb of the capital, Bujumbura.
At the time of the massacre, the FNL took responsibility saying its fighters had responded to shooting from within the camp. FNL forces were fighting to overthrow the Tutsi government of the day.
Rwasa ran for the top post in the central African country in 2010 but withdrew from the contest after accusing the ruling party of rigging the vote.
He later went into hiding saying he feared for his life. Burundi's authorities believe he fled to Congo for almost three years, though Rwasa says he stayed in Burundi.
Rwasa said on Friday he enjoyed immunity under the terms of a peace deal he signed with the government in 2006 until the establishment of a truth and reconciliation commission.
"The same peace deal states that only an international commission of inquiry is competent to qualify crimes of genocide, crimes against humanity and war crimes," Rwasa said.
But Agnes Bangiricenge, a spokeswoman for the prosecutor's office, told Reuters the immunity did not apply to the gravest charges. (Reporting by Patrick Nduwimana; Editing by Richard Lough and Sonya Hepinstall)