In November, more than 300 students and teachers were seized from a Catholic school. They were later released in two successive groups. It was among a spate of kidnappings that made international headlines.
Nigeria is facing numerous security challenges - including kidnappings for ransom by criminal gangs, an Islamist insurgency in the north-east, separatist violence in the south-east, and a battle between herders and farmers in the centre over access to land and water.
Experts say corruption, poor intelligence sharing and underfunded local policing have hampered efforts to tackle the various crises.
Nigeria's defence minister resigned last month at the height of the kidnapping crisis, officially for health reasons, according to the president's office.
The US has recently become militarily involved in Nigeria - launching airstrikes on Christmas Day on two camps run by an Islamist militant group in north-western Nigeria.
Earlier this month, US President Donald Trump warned of more strikes if Christians continued to be killed in the West African nation.
There are more than 250 ethnic groups in Nigeria, which is roughly divided into a mainly Muslim north, a largely Christian south, with intermingling in the middle - and the government says people of all faiths have been victims of attacks.
A Nigerian foreign ministry spokesman responded to Trump's warning by saying that Nigeria would continue to engage constructively with partners such as the US.
''Nigeria remains committed to protecting all citizens, Christians and Muslims alike, without discrimination,'' Alkasim Abdulkadir said.