Nigeria on Thursday opened the trial of five men suspected to belong to an Al-Qaeda linked terrorist group Ansaru, in connection with the mass kidnapping of schoolchildren earlier this year, a lawyer said.

The kidnapping in May of more than 40 schoolchildren and their teachers in Oyo state, in the country's relatively safe southwest, sent shockwaves through the country.

They were held captive for nearly two months before their rescue in an operation led by the Nigerian security forces.

The kidnapping was blamed on terrorists from Ansaru, a Boko Haram splinter group.

Pleaded not guilty

The five men pleaded not guilty to charges of complicity in committing acts of "terrorism" related to the mass abductions and killing of two workers.

The Nigerian military said its rescue operations, which lasted more than a month, targeted the kidnappers' wider networks and dismantled several hideouts in the forests of Old Oyo National Park.

Arrests across the country "completely disorganised the group, exerted overwhelming pressure on them and ultimately led the terrorist group to 'unconditionally release' the pupils and teachers", the military said when the pupils were freed.

The Oyo abduction prompted protests across the country and condemnation. Southwest Nigeria - home to economic capital Lagos - has long been considered one of the safest regions in a country dealing with multiple security threats.