At least 20 militants from Nigeria-based terror group Boko Haram have surrendered in Niger, the government has said. The fighters gave themselves up in the Diffa region, which has often been targeted by suicide bombing missions linked to the Islamist group.

Boko Haram started its violent insurgency in north-eastern Nigeria in 2009. However, the conflict soon spilled over into neighbouring countries, including Niger.

In December 2016, another 31 Boko Haram fighters surrendered in Niger.

"About 50 Boko Haram fighters have now given themselves up" since 27 December, Niger's interior minister, Bazoum Mohamed, said on the state TV channel Tele Sahel, according to the Vanguard newspaper.

Mohamed attributed the success to a coordinated military operation with Nigeria and Chad that took place last summer. He added that in some sectors, the war against the militants was "already over", but warned that some militants are still training in the Lake Chad basin area.

On New Year's Eve, Boko Haram fighters carried out an attack at Baroua, in the Diffa region, killing three Nigerien soldiers and wounding another seven. At least 15 militants also died.

The militants' surrender came days after Nigeria announced its army had stormed the group's last known stronghold in the Sambisa forest, in Borno state.

However, the claim was quickly dismissed by Boko Haram's contested leader, Abubakar Shekau, who accused Nigeria of spreading lies.

Boko Haram used to control territories the size of Belgium. However, Nigeria's ongoing military operation, Lafiya Dole, 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 Boko Haram 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/Daesh) would continue to pose major threats to stability in the region.

"There is no pressure for the Nigerian army and government to come out and say that they have defeated Boko Haram, whose priority now is not to win the war, but to survive and make sure they can inflict as much pain as damage to the people as possible," David Otto, counterterrorism expert at UK-based TGS Intelligence Consultants, told IBTimes UK.

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Who are Nigeria's Boko Haram?

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.