Nigerian President Muhammadu Buhari has laughed off rumours stemming from his ill health that he had died and been replaced by a clone or a lookalike impostor from Sudan, his spokesman said.

"It's the real me, I assure you. I will soon celebrate my 76th birthday and I will still go strong," Mr 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 impostor 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," Mr Buhari said, according to a statement signed by his spokesman Garba Shehu. 

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

Mr 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 Mr 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 Mr Buhari returned from another lengthy medical trip to London.

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