The United Nations said on Friday it was “extremely alarmed” by the forced return by Cameroon of thousands of refugees to north-east Nigeria, where Boko Haram Islamists pose a continuing threat to civilians.

“This action was totally unexpected and puts lives of thousands of refugees at risk,” the UN High Commissioner for Refugees (UNHCR), Filippo Grandi, said in a statement.

Cameroon forced “several thousand” refugees back to Nigeria this week, including 267 on Wednesday, the UN agency said.

“I am appealing to Cameroon to continue its open door and hospitable policy,” Grandi said, while calling on the government to immediately halt any more returns and meet its obligations under international law.

Cameroon has 370,000 refugees, 100,000 of whom are Nigerians, according to the UNHCR.

Profile Who are Boko Haram? Show Commonly known as Boko Haram, the Islamic State in West Africa is a terrorist organisation based in Northeast Nigeria. Formed in 2002 as Jamā'atu Ahli is-Sunnah lid-Da'wati wal-Jihād meaning “Group of the people of Sunnah for Dawa and Jihad”, the term Boko Haram is loosely translated as ‘Western education is forbidden’ or ‘Western influence is a sin’. Boko Haram started an armed rebellion against the Nigerian government in 2009. Their activities have included suicide bombings and the kidnapping of female students from a college in Chibok in 2014. A mass prison break-out in 2010 swelled their ranks. Their insurgency has also spread into neighbouring Cameroon, Chad and Niger.

In 2015 a coalition of troops from those three countries, alongside Nigerian forces, mounted a concerted effort to push back against Boko Haram. For their part, since 2015, Boko Haram has aligned itself with Islamic State. The UN estimates that at least 20,000 people have been killed in the conflict to date, but that is widely held to be far below the true number. At the peak of its strength it held territory equal to the size of Belgium Nigeria’s President Muhammadu Buhari and his generals have repeatedly declared that they have “beaten”, “defeated”, “technically defeated”, “routed” and “broken the heart and soul” of Boko Haram. Studies suggest casualties have dropped drastically in recent years. But the suicide bombings, child abductions and displacement of the local population continues. Estimates of the group’s strength vary between 4,000 and 20,000 fighters, and the number of people who have fled Boko Haram’s territory in the Lake Chad Basin is thought to be in the order of 2.4 million.

Earlier this month, more than 9,000 people fled to Cameroon after an attack on a military base and aid buildings in the town of Rann in north-east Nigeria’s Borno state.

The attack was blamed on the Islamic State West Africa Province (ISWAP) faction of Boko Haram, which has carried out similar raids against troops since July last year.

The upsurge in violence has forced tens of thousands of people to flee, including internally displaced people who have returned home after previous attacks.