LAGOS (Reuters) - Nigerian President Muhammadu Buhari will travel to Britain to see his doctor, he said on Monday in a surprise announcement that could renew concern about whether he is well enough to run for a second term at an election next year.

FILE PHOTO: Nigerian President Muhammadu Buhari addresses the 72nd United Nations General Assembly at U.N. headquarters in New York, U.S., September 19, 2017. REUTERS/Shannon Stapleton/File Photo

He spent five months in Britain last year for treatment for an unspecified illness, an absence that prompted many to question his fitness to run a country that is Africa’s top oil producer and one of its biggest economies.

Buhari, 75, was military ruler between 1983 and 1985 and returned to power after winning an election in 2015. He said in April that he would seek a second term, though his candidacy depends on party approval.

“I will be traveling to the United Kingdom tomorrow, to see my doctor, at his request. Will be away for four days; back in Abuja on Saturday, May 12,” he said on Twitter.

His spokesman said Buhari met his doctor last week in London on his way back from Washington where he had held talks with United States President Donald Trump in the White House.

“The doctor requested the president to return for a meeting which he agreed to do,” said the spokesman, Garba Shehu, in a statement.

Shehu, speaking to Reuters by telephone, said it would be a “routine check-up”.

Buhari’s prolonged absence from Africa’s most populous country sparked numerous protests last year, including demands that he should resign and calls for more transparency about his condition.

Any repeat of last year’s lengthy absence could hinder the president’s re-election campaign which got off to a shaky start after he drew criticism for saying many young Nigerians think they need “do nothing” because they live in an oil-rich country.

The president’s challenges include a surge in communal violence and high-profile attacks by Boko Haram militants that have turned security into a highly politicized issue.

The economy is also a key issue. Nigeria is only gradually emerging from a 2016 recession that was the country’s first in 25 years.

Last year he took two months’ medical leave in Britain starting in January and another break there from May to August.

He reduced his working day to a few hours after returning from his first stint of medical leave in March, diplomats and government sources said. And little was seen of him during his second stint in London.

Vice President Yemi Osinbajo acted as interim leader while Buhari was in Britain.