Opposition leaders in Democratic Republic of Congo lashed the third postponement in the country's presidential elections on Friday as a deadly plane crash added to the ballot's woes.An Antonov 26 chartered by the national electoral commission to deliver election material crashed on Thursday near Kinshasa, officials said.The plane, operated by the company Gomair, was about to land at the capital's airport after flying to the central city of Tshikapa where it delivered "election results forms," a spokesman at the Independent National Election Commission (CENI) said.Accounts of the numbers onboard and the casualty toll varied.An official with air traffic control in Tshikapa, the capital of Kasai district, said all six people onboard - five crew and one passenger - had died.But in Moscow, Russia's ambassador to the DRC, Alexei Senteboff, was quoted by news agencies as saying "sadly, it is confirmed that all three (crew members) died, and they are Russian citizens."Earlier, he said there were "unconfirmed figures" of 23 aboard, "including three crew members of Russian nationality".Peaceful handover Presidential, legislative and provincial elections were due to have been held in the vast, volatile country on Sunday, defusing a two-year-old crisis over the future of President Joseph Kabila.But on Thursday CENI ordered a week-long postponement, saying a fire at a warehouse last week had destroyed electronic voting machines and ballot papers earmarked for the capital.The delay will enable CENI to get replacement ballot papers from the South Korean firm supplying the equipment, the panel's chief, Corneille Nangaa, said."The electoral process continues," he promised.But in a country that has never experienced a peaceful handover of government since the end of Belgian colonial rule in 1960, suspicions of foul play run deep."Joseph Kabila does not want to leave power or organise elections," an alliance of parties backing opposition candidate Felix Tshisekedi declared on Friday. "Any further postponement, even a single day" beyond the new date of December 30 "will not be accepted", the coalition, CACH, warned.Martin Fayulu, another opposition frontrunner, said: "No delay can be justified."* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook. Kabila and Nangaa will bear "full responsibility for this charade," he said on Thursday.His statement was also signed by two major figures who have been barred from contesting the election - the exiled former governor of Katanga province, Moise Katumbi, and former warlord Jean-Pierre Bemba.And this year's Nobel Peace laureate Denis Mukwege called for Kabila and "his corrupt group of cronies" to resign and be replaced in an op-ed published Friday in the New York Times, warning there would be greater instability, violence and chaos if he clung to power."Congo desperately needs a technocratic caretaker government to reorient the government while overseeing the holding of free and fair elections," Mukwege wrote.Kabila's future There was no report of any violence in the aftermath of CENI's announcement. Dozens of people have been killed by the security forces in anti-Kabila protests in the past two years.A long history of political turmoil, bloodshed and dictatorship explains the tension behind the elections.Since 1996, the Democratic Republic of Congo has suffered two major wars that left millions of dead, and two ongoing conflicts in the centre and east of the country that have caused hundreds of thousands to flee their homes.In 2016, the spectre of instability rose once more as Kabila kept the nation guessing about his future.Kabila took office in 2001 after succeeding his assassinated father.Kabila, 47, was due to step down at the end of 2016 after reaching the end of his constitution-limited two terms in office.But he stayed on, invoking a caretaker clause in the constitution that enables a president to stay in office until his or her successor is elected.The elections were postponed until the end of 2017 under a deal brokered by the powerful Catholic church - and then again until 2018, when Kabila eventually confirmed he would not run again.The delay sparked protests that were bloodily suppressed, causing dozens of deaths and sparking an outcry from western nations and the UN.One of the figures behind the crackdown is Kabila's hand-picked successor, Emmanuel Ramazani Shadary, a hardline former interior minister.Critics of the president say Shadary will be manipulated from behind the scenes or set the stage for Kabila to make a comeback.