Nigeria's president took the extraordinary step of denying rumours that he died and was replaced by a body double, telling the country that he is alive and well."It is the real me I assure you," President Muhammadu Buhari said on Sunday to a group of Nigerians during a visit to Poland, where he is attending the United Nations Climate Conference."A lot of people hoped that I died during my ill health," Buhari said. "I am still going strong."The 75-year-old, who was elected in 2015 and will run for his second term in February, has been in ill health throughout his presidency. But in the video of his remarks posted to Twitter by his personal assistant, he joked as he dismissed the rumors, to laughter and head-shaking applause by some government officials after a Nigerian posed a question about his identity.The government has been tight-lipped about Buhari's health throughout his presidency.Rumors of his death started in 2017, when Buhari spent seven weeks in London for medical treatment. They abated when he returned to Nigeria, but returned in full force last month, stoked by prominent opposition leaders and separatists.Suicide bombings and abductions Nnamdi Kanu, leader of the separatist group, Indigenous People of Biafra, said that a Sudanese lookalike, named Jubril, had taken Buhari's place as a body double. His claims were shared widely online, often accompanied by videos that appeared to portray a dead Buhari lying in a London hospital.This is not the first time that Nigerians have speculated about a president's mortality. State secrecy around former president Umaru Musa Yar'Adua's ill health fostered similar rumors before Yar'Adua died in office in 2010.As Nigeria prepares for the upcoming elections, the opposition said the president's health renders him unfit to continue his tenure. Some have questioned his ability to contain insurgent groups and Islamic extremists, which the president recently said had started using drones.In the 2015 elections, Buhari made the defeat of Boko Haram a major goal for his presidency. But extremists continue to carry out deadly suicide bombings and abductions in the northeast and wider Lake Chad region.Deadly attacks against the Nigerian military are on the rise. Thirty-nine Nigerian soldiers were killed and another 43 were wounded in November, according to the presidency.In late November, Buhari pivoted from repeated claims from his government that Boko Haram had been "crushed," instead urging the military to "rise to the challenge."The Islamic State also claimed it has "full control" of Arege, a town near Lake Chad, after Nigerian soldiers fled their barracks.The group also posted a statement Monday on the main IS website claiming that, after two days of attacks against the barracks, many soldiers were killed and wounded.The Nigerian military has not responded to the claims.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook. Buhari said Monday at the UN Climate Change conference in Poland that no country can fight climate change on its own and called for international support to save the receding Lake Chad and ensure its safety from Boko Haram fighters."Nigeria believes in joint and co-operative effort to tackle the problem," Buhari said, noting that climate change effects are felt more acutely in vulnerable communities. "We urge that effort to address the challenges of climate change be pursued within multilateral frame work."He said parameters should be put in place to monitor financial flows from developed countries to developing economies.Lake Chad, one of the world's largest freshwater lakes, has shrunk by 95 percent in more than 50 years, the government says. Buhari has said that has led to massive social and economic loss for millions of families, and has linked it to violence by Boko Haram insurgents.He called on the international community to support a water transfer from the Congo Basin, saying it would benefit of over 40 million people that depend on the Lake Chad for their livelihood and to guarantee future security of the region.