Nigeria's President Muhammadu Buhari has denied claims that he had died and been replaced by a Sudanese impostor, quashing a rumour that has circulated on social media for months.

"It's the real me, I assure you. I will soon celebrate my 76th birthday and I will still go strong," Buhari told Nigerians in a town hall session in Poland on Sunday, where he was attending a conference.

"A lot of people hoped that I died during my ill health," he said, adding that those who spread the rumour were "ignorant and irreligious".

The Nigerian leader, who is running for re-election in February, spent five months in the United Kingdom last year being treated for an undisclosed illness.

One theory widely aired on social media - and by some political opponents - was that he had been replaced by a lookalike from Sudan called Jubril.