Nigerian president Muhammadu Buhari has shocked his country by announcing he is heading to London for medical checks

The news came immediately after he met a group of 82 Chibok girls freed from Boko Haram extremists.

He has missed three weekly cabinet meetings and he spent about six weeks in London on medical leave earlier this year, later saying that he had never been as sick in his life.

The cause of his illness is unclear but a statement from his office said there was "no cause for worry" and that the length of his London stay would depend on advice from doctors.

A thin-looking Mr Buhari, 74, had welcomed the Chibok girls at his official residence on Sunday evening, a day after they were released.

The girls were released in exchange for five Boko Haram fighters who were being held by Nigerian authorities, as part of a deal brokered by the International Committee of the Red Cross and the Swiss Government.

They are now with government officials in Nigeria's capital Abuja, where they will be rehabilitated before returning to their parents, many of whom do not know whether their daughters are among those freed.

Pernille Ironside, acting representative of Unicef Nigeria, "These girls have faced horrific and unspeakable crimes and experiences.

"It is now our collective job to support the government and the families of those girls to be able to take that important next step forward."

The girls will be kept in Abuja until they are returned to their families

The rehabilitation process has previously been criticised, mainly for keeping the girls in Abuja, around 560 miles from their families in Chibok.

Amnesty International's Nigeria office said the girls should not have to endure a lengthy government detention or a "publicity stunt" but instead should be given privacy to recover.

Aisha Muhammed-Oyebode, of the Murtala Muhammed Foundation, which helped the first group of students, said she hoped the girls would be able to see their families during the rehabilitation process.

"I think one of the greatest challenges will be being able to redefine what is going to be normal for them-Aisha Muhammed-Oyebode

"These girls have been in captivity for three years and they've gone through very difficult, challenging and horrendous experiences.

"Many of them have come back with children, babies. They went away as young girls trying to get an education and most of them have come back as women."

There are still 113 girls missing, of the 276 abducted in 2014.

According to the girls who already escaped, some of their classmates have died from illness, some do not want to return home, having been radicalised by their captors, and some may have been used to carry out suicide bombings.