The departure of Mohammed Badaru Abubakar came after President Bola Tinubu declared a "nationwide security emergency" last week as the country scrambled to respond to a wave of mass kidnappings that have seen hundreds of people, mostly schoolchildren, captured within days last month.

Tinubu's spokesman, Bayo Onanuga, said in a statement that Abubakar, 63, was quitting with immediate effect, on health grounds.

"His resignation comes amid President Tinubu's declaration of a national security emergency, with plans to elaborate on its scope in due course," the spokesman said.

Africa's most populous country has long experienced insecurity but the spate of abductions recently has left it scrambling.

US President Donald Trump in late October named Nigeria a Country of Particular Concern (CPC) -- a State Department designation for religious freedom violations -- over what he called killing of Christians by "radical Islamists". He subsequently threatened to intervene militarily.

Washington's rhetoric -- rejected by the Nigerian government and independent security analysts -- has placed the country's security crisis under the spotlight.

Armed gangs seized more than 300 teachers and staff at St. Mary's co-education school in north-central Nigeria on November 21. Fifty escaped but the rest are still in captivity.

"The children are fine and will be back soon," national security adviser Nuhu Ribadu, was quoted as saying during a high-profile visit with school officials in the town of Kontagora, in central Niger state.

- Gangs, jihadists -

Since Boko Haram jihadists kidnapped nearly 300 schoolgirls from the northeastern town of Chibok in an infamous raid more than a decade ago, Nigeria has struggled to contain mass kidnappings.

In addition to kidnappings, which are frequent in Nigeria and are mostly carried out by criminal gangs seeking quick ransom payments, Africa's most populous country has been battling a deadly jihadist insurgency in its northwestern regions, since 2009.

In the wake of the kidnappings, the country's president declared emergency, and ordered mass recruitment of police and military personnel.

Recent raids have resulted in kidnappings of schoolchildren and teachers, worshippers and priests, a bride and her bridemaids, farmers, women and children as well as farmers across various parts of the country.