What illness he has remains undisclosed but concerns about his condition grew in recent weeks after he missed several cabinet meetings.
Boko Haram has used kidnapping as a weapon of war, seizing thousands of women and children, and forcibly recruiting young men and boys into their ranks.
"The length of the President's stay in London will be determined by the doctors". The vice president will be in charge.
"I can not express in a few words how happy I am to welcome our dear girls back to freedom", Mr Buhari told the girls in Abuja, according to his office. A thin-looking Buhari met the girls in the evening.
Other Nigerians including some parents of the kidnapped girls gathered in Abuja at the Unity Fountain to celebrate the weekend release. He says the president promised they will continue with their education.
United Nations officials have stressed that the Chibok girls are not Boko Haram's only victims.
The girls were welcomed by the Nigerian president's chief of staff.
Enoch Mark, a Christian pastor whose two daughters were kidnapped in 2014, told Agence France-Presse: "This is good news to us".
The girls have been ferried to Abuja and are now awaiting a formal reception by President Muhammadu Buhari.
Some are thought to have been sold as brides to Boko Haram fighters at 2,000 naira (£4) each. We assure of government's resolve to give comprehensive medical attention to the released schoolgirls and ensure their rehabilitation and reintegration into normal society.
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Morissette told a Los Angeles judge Wednesday that Schwartz "stole a dream", the Canadian Broadcasting Corporation (CBC) reported. Jonathan Todd Schwartz, 48, admitted to stealing the money as part of a scheme to pay for his lavish lifestyle, officials said.
Five Boko Haram commanders were released in exchange for the girls' freedom, a Nigerian government official said Sunday.
There was no comment yet from the Nigerian presidency or Boko Haram, an extremist group linked to the Islamic State.
Receiving the girls Buhari said their return is a pleasant 2nd anniversary gift to the people of Nigeria.
The 82 girls were released to global negotiators who have been working in collaboration with the Federal government for their safe return since they were kidnapped in April 2014. Almost 60 girls who escaped during the first hours said their abductors forced them from dormitories into trucks that headed into the bush. Others did not want to come home because they'd been radicalized by their captors, they said.
But the discovery of one of them with a baby last May raised hopes for their safety, with a further two girls found in later months and a group of 21 released by the Islamist militants in October.
"Nigerians stood together as a people and purposed to free these girls", he says.
The newly released schoolgirls were set to meet with President Muhammadu Buhari in the capital later Sunday.
Boko Haram descended on Damasak on November 24, 2014 and honed in on Zanna Mobarti Primary School, locking more than 300 students, ages 7 to 17, inside.
"Our security agencies have taken back these abducted girls in exchange for some Boko Haram suspects held by the authorities", he said.
In response to a question, the source said: "Both parties agreed to manage the release of the girls in a no victor, no vanquished manner". They will meet Buhari at the presidential villa.
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