Nigeria’s President Muhammadu Buhari is seeking fresh treatment in London for an undisclosed illness.
His health has been a major cause of concern in a country where there are fears that a power vacuum could affect its recovery from recession.
In a brief message, the president said “there is no cause for worry”.
Mr Buhari, 74, has left Vice-President Yemi Osinbajo in charge, just as he did when he left for London in January for seven weeks of medical leave.
Mr Osinbajo was widely praised at the time for his performance as acting president.
The exact length of the president’s stay in London “will be determined by the doctors”, a statement from the presidency said.
But, it added, “government will continue to function normally under the able leadership of the vice-president”.
His last official act before leaving for London was to meet the 82 schoolgirls kidnapped in Chibok in 2014 after they were released by Boko Haram Islamist militants.
The girls were handed over on Saturday in exchange for five Boko Haram suspects after negotiations, sources have told the BBC.
A list of the girls’ names has been published, but it is not clear if all the parents have been formally notified, whether or not their girls are among those released.
They were from a group of 276 abducted from their school in north-eastern Nigeria. About 113 of the girls are still missing, along with hundreds of other people kidnapped by the militant group.
Nigerians are particularly sensitive to the health of their president after then President Umaru Yar’Adua sought medical treatment in Saudi Arabia in 2009.
His failure to hand power to the vice-president and the lack of information about his condition led to widespread anxiety. He died in office in 2010.
When Mr Buhari returned from London in March, he said he had never felt “so sick” as he had when he was being treated and warned that he may have to undergo further medical checks.
Concerns over his health were rekindled after he missed the last three cabinet meetings, prompting civil society activists to urge him to return to London for further treatment.
His appearance at Friday prayers last week was the first time he had been seen in public for two weeks.
His aides said he had been resting and working from home and the president’s wife Aisha Buhari said he was not as sick as people thought.
Buhari’s unhealthy start to 2017
19 January – Leaves for UK on “medical vacation”
5 February – Asks parliament to extend medical leave
10 March – Returns home but does not resume work immediately
26 April – Misses second cabinet meeting and is “working from home”
28 April – Misses Friday prayers
3 May – Misses third cabinet meeting
5 May – Appears at Friday prayers in Abuja
7 May – Travels to UK for further treatment
Nigeria Chibok girls: Parents learning if daughters among those freed
Parents of the missing Chibok girls are slowly learning if their daughters are among the 82 freed by Boko Haram militants in Nigeria two days ago.
The girls’ names were put on Twitter by the president’s office on Sunday. They were flown to the capital Abuja.
But in Chibok, their home region in north-eastern Nigeria, not everyone has access to the social media site.
It is unclear if the government has made other attempts to let them know if their daughters are now safe.
On Monday, people were checking the newspapers to see who was on the list and decide whether to make the journey to Abuja, according to the Associated Press news agency.
Even parents in Abuja – where the 82 girls were flown in order to meet President Muhammadu Buhari before he left the country for medical treatment – were waiting to see if they would be reunited with their daughters.
Esther Yakubu told the BBC the last three years had been a “horrible nightmare” but that even the possibility of her daughter having been rescued was giving her hope.
“Whether she is among the freed ones or not, I am very happy,” she said. “We started this year with 24 [returned girls] and now we have 106. It is a large number, and we have hope that, if they are alive, they will come back.
“I have never been happy in my life like today. I am a mother. I accept any child that is back. My baby will be back soon, if she is among them or if she isn’t.”
It is being reported that the girls were handed over on Saturday in exchange for five Boko Haram suspects after negotiations – a deal which has been criticised by the Peoples Democratic Party (PDP), led by Senator Ahmed Makarfi.
In a statement, the PDP faction said the exchange had allowed terrorists to escape punishment and would embolden them to carry out further kidnappings, while the “piecemeal” release of the girls meant they still held bargaining chips.
Boko Haram is thought to still be holding more than 100 of the original 276 girls taken from a school in north-eastern Nigeria in 2014.
However, they are far from the only people abducted by the extremist group. Amnesty International has recorded 41 cases of mass abductions in the last three years. It puts the number of women and children kidnapped at at least 2,000.