Nairobi - Senior leaders of the Popular Forces of Burundi (FPB) rebel group were arrested in eastern Tanzania on Saturday and extradited to Burundi the following day, the FPB said on Monday."Four FPB leaders, including the number 1 and 2, Jeremie Ntiranyibagira and Edouard Nshimirimana, were arrested in Ngara on October 21 by security forces from Tanzania and Burundi," it said in a statement."On October 22, they underwent an irregular extradition to Burundi, where their lives are in danger," it said, giving no further details.The statement was authenticated as genuine by several senior figures in the FPB, the name given to the Republican Forces of Burundi - a rebel militia active in eastern Democratic Republic of Congo that opposes Burundi's president, Pierre Nkurunziza.The claim made in the communique could not, however, be immediately confirmed by Tanzanian or Burundian official sources. But Ikiriho, a news site close to Burundi's government, denied on Twitter that any extradition had taken place.The Republican Forces of Burundi mainly comprises soldiers and police who fled following a failed coup.In August this year, the group said it was renaming itself the Popular Forces of Burundi - FPB, under its French acronym - under the leadership of General Jeremie Ntiranyibagira, with Colonel Nshimirimana as his deputy.A senior source in the PFB, speaking on condition of anonymity, said the two "went on a mission to Tanzania and fell into a trap."Their arrest was "a terrible blow," the source added."We think that the Burundian authorities are hiding for the time being that they were sent immediately to Burundi, in order to be able to torture them and gain information on our movement," the source said.Burundi is a poor, small central African nation marked by a troubled ethnic history and a deadly civil war.Its most recent crisis began in April 2015, when Nkurunziza, in office since 2005, said he planned to run for a third term. The plan was dubbed anti-constitutional by civil society, the opposition, the Catholic church and even some of his own supporters.Protests gathered steam, which were then banned by the authorities and were followed by a bloody crackdown.In May 2015, the authorities foiled a military coup but some of the putschists got away.Between 500 and 2 000 people died in the crisis, according to varying tolls from the UN and NGOs, while hundreds of people have disappeared or been tortured and more than 400 000 people have fled their homes.According to a report by UN experts on DRC, the FPB is the biggest Burundian rebel group, in numerical terms. It is estimated to have between 300 and 500 fighters, based in South Kivu, eastern DRC.