Police in Nigeria say they have arrested 20 suspected armed kidnappers in the country's northwest, where kidnappings for ransom are common.

Authorities said the kidnappers belonged to two separate gangs that mainly operated on the region's main highways.

In a statement, police spokesman Frank Mba said "sophisticated" weapons, including 11 large caliber grenades and 11 rifles, had been recovered.

While they are yet to appear in court, the statement said the main suspect sometimes disguised himself as a commercial bus driver. He says he transported passengers from car fleets and briefed gang members who then ambushed themselves along the road to kidnap unsuspecting travelers for ransom.

Nigeria is grappling with a growing wave of murders and kidnappings for ransom by armed criminal gangs targeting schoolchildren, communities and travelers.

Despite reports of numerous arrests of suspects by the authorities, prosecutions and convictions are very rare.