Nigeria's President Muhammadu Buhari is "recuperating fast" from his illness, his wife Aisha Buhari has said.

She made the comments after a trip to London, where her husband is being treated for an unspecified illness.

Mr Buhari, 74, left for London nearly a month ago - his second trip to the UK for treatment this year.

His absence has led to some anxiety in Nigeria, Africa's biggest economy, with people speculating about whether he will be able to resume his duties.

Vice-President Yemi Osinbajo is acting president, and has been widely praised for his handling of the country's economic crisis, following a sharp fall in the price of oil, its major export.

Mr Buhari took about seven weeks of medical leave in January, and flew to the UK for treatment.

When he returned home in March, he hinted that he had undergone a blood transfusion, but did not disclose his illness.

He flew again to London on 7 May for further treatment, and has not indicated when he will return.

Mrs Buhari said her husband thanked Nigerians for their "constant prayers and said he will soon be with them as he is recuperating fast".

She returned to Nigeria after spending about a week with her husband in London.

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 consecutive cabinet meeting

5 May: Appears at Friday prayers in Abuja

7 May: Travels to UK for further treatment

6 June: Aisha Buhari says his is 'recuperating fast'