Nigerian president's month-long medical leave overseas has empowered his deputy to attack the country's problems with an energy that has surprised observers and led some to suggest he should stay in charge for good.

When he left for London on Jan. 19, President Muhammadu Buhari handed power to Vice President Yemi Osinbajo, a 59-year-old lawyer and pastor. He has embarked on a whirl of domestic travel and meetings, taking on long-standing, seemingly intractable issues like the weakened naira currency and oil militants whose bombings have cost the country billions of dollars.

That isn't silencing some critics who say Buhari's absence has caused a power vacuum, nor calming the concerns of citizens amid an official silence on the cause of the president's leave. On Tuesday, the 74-year-old Buhari sent a message thanking Nigerians for their prayers and saying "I need a longer time to rest."

Suggestions that Buhari is too incapacitated to run one of Africa's largest economies and its No. 2 oil supplier have prompted a few calls for his resignation.

Citizens' advocacy group Opinion Nigeria cited the country's many challenges in calling for "the immediate resignation of President Muhammadu Buhari ... followed swiftly with the immediate confirmation of Vice President Yemi Osinbajo as a substantive president."

But putting Buhari's deputy in charge for good could provoke antagonism between majority Christian southerners like Osinbajo and mainly Muslim northerners like Buhari. When former President Musa Yar'Adua was ill abroad for months before coming home to die in 2009, northerners blocked his southern Vice President Goodluck Jonathan from assuming power, creating a months-long political paralysis.

Jonathan was eventually confirmed, but his subsequent successful run for election angered many Muslims, breaking an unwritten agreement that power rotate between northerners and southerners.

That scenario is unlikely to be repeated now, a Eurasia Group risk analysis wrote this week. Buhari's "practice of handing over authority to Vice President Yemi Osinbajo in his absence would probably facilitate a calm transition if Buhari is unable to complete his term",  it predicted.

The analysis said Osinbajo, who is new to politics and has no national power base, likely would not run in 2019 presidential elections.

This week, the Daily Post newspaper quoted presidential spokesman Garba Shehu as denying that "some close appointees of President Buhari had constituted themselves into a cabal with the aim of whittling down the influence of Acting President Osinbajo."

Osinbajo last week presided over an hours-long meeting where the National Economic Council demanded urgent action on the naira's freefall. This week, the Central Bank offered a devalued naira rate to those who can afford medical services and education abroad. It is seen as a test for a further devaluation of the currency.

Buhari had long resisted pressure to devalue the naira until Osinbajo announced the move last year during another of the president's medical leaves.

When Nigerians protested earlier this month against rising food and fuel prices, ongoing corruption and a feeble government, Osinbajo did not send troops to break up their march, as had happened in the past. Instead, he met with labor leaders and enlisted their help in fighting corruption and inflation running near 20 percent.

"We hear you loud and clear", he said in a message to protesters.

Also this month, Osinbajo traveled to the heart of oil militant country in the south, where he declared that young men must be provided proper jobs to stop militancy and that the government will work to transform illegal oil refineries - often the only source of fuel in impoverished villages - into legal ones.

Osinbajo said he was there as an emissary of Buhari to offer "a new vision" for the oil-producing Niger Delta, where militant attacks resurged after the president's government tried unsuccessfully to halt payments under a 2009 amnesty program.

Buhari has never visited the region himself.

AP