Nigerian President Muhammadu Buhari on Sunday quashed a rumour stemming from his ill health that he had died and been replaced by a lookalike impostor from Sudan, his spokesman said.

"It's real me, I assure you. I will soon celebrate my 76th birthday and I will still go strong," Buhari said as he met with the Nigerian diaspora in Poland, where he is attending the UN COP24 climate summit in Katowice.

He was answering a question from the audience about repeated claims -- spread via tweets, Facebook posts and YouTube videos -- that the leader of Africa's most populous nation was an imposter called "Jubril".

"A lot of people hoped that I died during my ill health. Some even reached out to the Vice President to consider them to be his deputy because they assumed I was dead," Buhari said, according to a statement signed by his spokesman Garba Shehu.

He referred to those who started the rumour as "ignorant" and "irreligious".

Buhari, who is seeking re-election next year, spent a large part of 2017 in London for treatment for a serious illness, which has never been revealed to the public.

A lack of specific information about the illness, along with Buhari's gaunt features and a reduction in public appearances have fed speculation about his well-being.

Claims about the president's identity emerged a month after Buhari returned from another lengthy medical trip to London.

Those pushing the rumour were known critics of the president and his government.