The Boko Haram jihadist group released a video on Thursday claiming to show schoolboys seized in a mass kidnapping in northwest Nigeria last week.

A distraught teenager, speaking in English and Hausa in the video seen by AFP, said he was among 520 students taken by "the gang of Abu Shekau."

The teenager was surrounded by a large group of boys -- some looking very young -- who were clustered under a tree, appearing grubby and exhausted.

The assault last Friday on a rural school in Kankara, Katsina state, was initially blamed on criminals, known as bandits, who have terrorised the region for years.

But on Tuesday Boko Haram claimed responsibility for the raid, which occurred hundreds of kilometres (miles) from its stronghold in northeast Nigeria -- the birthplace of a brutal decade-old insurgency.

The video was released with a recording of a voice ressembling that of the group's elusive leader, Abubakar Shekau.

It reiterated the claim of responsibility.

A girl coated in oil sits in a canoe as she sorts out crabs near the River Nun in Nigeria's oil state of Bayelsa November 27, 2012. REUTERS/Akintunde Akinleye

"I earlier released an audio confirming our people did God's work, but people denied it," the voice said. "Here are my men, and your children have spoken."

The video was sent to AFP via the same channel as previous messages by Boko Haram.

Security sources told AFP Wednesday that the operation was carried out on Boko Haram's orders by a notorious local gangster called Awwalun Daudawa, in collaboration with Idi Minorti and Dankarami, two other crime chiefs with strong local followings.

The government has not immediately reacted to Boko Haram's claims, nor confirmed the exact number of children missing.

Two accounts by different officials have put the number of schoolboys at 320 or 333.

Shekau was behind the 2014 abduction of 276 schoolgirls in Chibok that sparked global outrage.

#BringBackOurBoys started trending on social media, in reference to a similar hashtag after the Chibok kidnappings.

Small protests to push for the boys' release took place in Katsina as well as in the capital Abuja on Thursday.