The raid on the Nigerian Defence Academy, the country's main officer training school, is a major blow for a military already struggling with a jihadist insurgency and heavily-armed criminal gangs.

"The security architecture of the Nigerian Defence Academy was compromised early this morning by unknown gunmen," said Major Bashir Muhammad Jajira, spokesman for the academy in the northwestern state of Kaduna.

"We lost two personnel and one was abducted."

An internal army message seen by AFP said a lieutenant commander and a lieutenant were killed and a major was abducted. Another officer was also wounded.

The high-security base, located just outside the state capital Kaduna, trains Nigerian officers and also cadets from other African militaries.

The academy statement said only that the attack had been carried out by "unknown gunmen".

No group claimed responsibility, but Nigeria is facing a threat from jihadists and large criminal gangs that raid villages, steal cattle and carry out mass kidnappings for ransom.

Ansaru, a splinter group of Boko Haram which is linked to al-Qaeda, is also present in the northwest, specialising in high-profile kidnappings.

However, the group has mostly been inactive since its leadership was hit hard by security forces in recent years.

- 'Bandits' threat -

The criminal gangs, also known as bandits, often attack in large numbers and arrive on motorbikes.

Typically motivated by financial gain, they have been targeting schools and colleges, kidnapping students and pupils for ransom.

The armed forces have carried out operations and air strikes on their camps, which are hidden deep in the forests that span Zamfara, Kaduna, Katsina and Niger States, but violence has escalated.

In July, the air force said bandits had shot down one of its jets as it was carrying out operations in Zamfara state. The pilot ejected and escaped.

President Muhammadu Buhari, a former soldier first elected in 2015, has warned paying ransoms will provoke more kidnappings. Some local governors have tried to negotiate amnesty deals to stop the attacks, though those accords have mostly failed.

In the northeast of the country, there are signs of growing ties between the bandits and Islamic militants -- a government document this month warned bandits were getting training from jihadists.

Nigeria's 12-year-long Islamist insurgency has left over 40,000 people dead and forced around two million more from their homes in the northeast where another Boko Haram splinter group, the Islamic State West Africa Province (ISWAP), is now the dominant armed group.

Boko Haram jihadist chief Abubakar Shekau was killed earlier this year during infighting with ISWAP militants in a major shift in the grinding conflict.