Nigeria's President Muhammadu Buhari missed his third consecutive cabinet meeting on Wednesday because he needed to rest, according to officials, raising concerns about the state of his health.

Vice President Yemi Osinbajo chaired the start of the session, which was open to the media. Buhari has been resting after treatment in the United Kingdom for an unspecific illness.

Information Minister Lai Mohammed said the president had been in the office on Tuesday.

"He did not attend because he took today to rest," Mohammed told reporters on Wednesday. "All he is doing is following his doctors' advice to rest enough."

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Meanwhile, the president's wife sought to play down fears about her husband's health as Buhari faced calls to go on extended medical leave.

Aisha Buhari said the head of state's health was "not as bad as it's being perceived", without specifying the nature of his illness.

"He continues to carry out his responsibilities during this period," she wrote on her Twitter account on Tuesday evening.

Buhari, 74, has been under growing pressure to disclose his state of health since he returned from London in early March after nearly two months away.

He missed two cabinet meetings in April, was absent from Friday prayers last week, and failed to attend his grandson's wedding on Saturday.

His spokesman said that he had undergone "a long period of treatment" in London and needed rest.

"Concerns are growing now that Buhari again was not seen at the federal executive council meeting," Al Jazeera's Ahmed Idris, reporting from Nigeria's capital, Abuja, said.

"Already there are some activists and prominent Nigerians who are asking the president to transfer power to his deputy and go and consult his doctors for proper medical consultation and treatment," he added, referring to an open letter signed by 13 civil society leaders on Tuesday.

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Nigerian media reported on Wednesday that former president Olusegun Obasanjo met other ex-military rulers Ibrahim Babangida and Abdulsalami Abubakar to discuss the situation.

The health of Nigeria's president has been a sensitive issue since the death in office in 2010 of Umaru Musa Yar'Adua, which sparked months of political turmoil.

During the 2015 election campaign, Buhari rejected opposition claims that he was seriously ill with prostate cancer and said that they were a smear to show him as unfit for office.