Nigeria's Minister of Defence, Mohammed Badaru Abubakar, has resigned from his position with immediate effect for health reasons, the president's office has said. 

The 63-year-old's departure coincides with a period of heightened security challenges across Nigeria, with the government under pressure to deal with a spate of mass kidnappings.

Last week, the office of the UN High Commissioner for Human Rights reported that at least 402 people, mostly schoolchildren, had been kidnapped since mid-November.

In the latest sign of the insecurity, gunmen abducted at least 20 people, including a Christian pastor, a Muslim bride and her bridesmaids, in two separate raids in the north on Sunday.

Some 250 schoolchildren and 12 teachers from a Catholic school in central Niger state are still believed to be missing following the biggest mass kidnapping attack in recent weeks.

It is not clear who is behind the kidnappings - most analysts believe they are carried out by criminal gangs seeking ransom payments, however a presidential spokesman has told the BBC they believe they are the work of jihadist groups.

Abubakar was appointed to his post in August 2023, and it is unclear what health issues prompted him to resign. 

He wrote a letter to President Bola Tinubu, informing him of his decision. 

Tinubu's office said the president accepted his resignation, and thanked him for his "services to the nation". 

It also noted that Tinubu had declared a national security emergency, with plans to bolster the police force to about 50,000 by recruiting to 20,000 more officers.

Nigeria is facing numerous security challenges - including kidnappings for ransom by criminal gangs, an Islamist insurgency in the north, separatist violence in the south-east, and a battle between herders and farmers in the centre over access to land and water.

Abubakar served as a two-term governor of the northern state of Jigawa from 2015 to 2023.

He was an important figure in Tinubu’s presidential election campaigns just over two years ago, helping him win his native Jigawa state. He was rewarded with the defence portfolio when Tinubu took office. 

Tinubu's spokesman said the president is expected to inform the Senate of Abubakar's successor later this week.