The Nigerian military has mistakenly killed 20 aid workers and more than 100 refugees during an operation against Boko Haram terrorists in Borno state.

Officials have admitted that one fighter shot refugees and members of the International Committee of Red Cross (ICRC) and the Medicines Sans Frontiers, the first time it is believed military sources have admitted such a mistake.

More on Nigeria

Authorities in Borno state, the heartland of Boko Haram, confirmed to Associated Press that more than 100 refugees had been killed. A Red Cross worker said 20 volunteers with the aid group had been killed.

"This morning, we received a report about the gathering of Boko Haram terrorists around Kala Balge area of Maiduguri. Unfortunately, the strike was conducted but it turned out that other civilians were somewhere around the area, and they were affected," Major General Lucky Irabor, the theatre commander of anti-terror operation Lafiya Dole, was quoted by the Vanguard newspaper as saying.

"So far, it is a little bit disturbing; death has occurred. There are casualties; there were deaths and injuries, but on the actual number of casualties, we would get back to you later.

"I am yet to get the number of casualties of civilians killed, but two soldiers were also affected. Some humanitarian staff of Medicines Sans Frontiers and some staff of ICRC were also affected. We are sending helicopters to evacuate those that were critically wounded, including our wounded soldiers," Irabor continued.

The statement was issued hours after Boko Haram's contested leader Abubakar Shekau claimed responsibility for twin suicide bombings that killed at least four people in the country's north-east. The suicide bombings occurred at the Maiduguri university in Borno on Tuesday (16 January 2017).

In the clip posted on YouTube, Shekau also denied claims that the army had killed militants in their stronghold in Bonro's Sambisa forest.

Is the fight against Boko Haram over?

Boko Haram used to control territories the size of Belgium. However, Nigeria's ongoing military operation LafiyaDole, and a regional offensive – consisting of 8,700 troops from Nigeria, Niger, Chad, Cameroon and Benin – have scored some success, with soldiers recapturing key territories and releasing thousands of civilians held captive by the group.

Security analysts have pointed out that declaring a victory over the group is premature, given that BokoHaram is still able to carry out attacks and recruit people. Experts also warned that underlying issues such as disenfranchisement, poverty and strong links with Islamic State (Isis) would continue to pose major threats to stability in the region.

Nigeria up close: Check out our Flipboard magazine

Who are Nigeria's Boko Haram terrorists?

Boko Haram, which has renamed itself Iswap, fights against Western influence in Nigeria and aims to impose its version of Sharia law throughout occupied territories.

The group launches attacks in Nigeria and neighbouring countries in a bid to take control of more territory. Three Nigerian states − Adamawa, Borno and Yobe − have been under a state of emergency since May 2013.

Boko Haram has killed more than 20,000 people since 2009 and was deemed the world's deadliest terror group, surpassing Islamic State in November 2015. Nigeria has also become the world's third-most terrorised country as a result of the group's violent insurgency.

Boko Haram Isis