Gunmen in Nigeria attacked a bus in the country's northwestern Sokoto state, causing a fire that killed 23 people, police and the governor's spokesman said Wednesday.

It was unclear why the bus was attacked but heavily armed criminals, known locally as bandits, have for years terrorized northwest and central states in Africa's most populous nation.

"23 people lost their lives," said the governor's spokesman Muhammad Bello.

The bus was traveling from the small village of Gidan Bawa in Sokoto, near the border with Niger, down to Kaduna state on Monday, said Bello.

"Of the total number of people in the bus which the bandits shot at causing a conflagration, 23 died of fire burns," the statement added, quoting the state police commissioner, Kamaldeen Okunola.

Six others sustained minor injuries and are receiving treatment, he added.

"Security agencies mobilised to the spot of the attack and immediately mounted a patrol that is still ongoing," said Okunola. 

Since last year, bandit gangs have become more violent.

This year alone, more than 1,400 children were kidnapped by bandits according to UNICEF.

Since January 2020, more than 57,000 people fled from their homes in northwest and central Nigeria due to insecurity, according to the International Organization for Migration. 

More than 80,000 additional people have fled to neighboring Niger over the past two years.