At least 10,000 Nigerians returned to a flashpoint town on Wednesday after fleeing into Cameroon to escape repeated attacks by the militant group Boko Haram over the past few months, according to a UN humanitarian report.

The returnees were among 40,386 civilians who had fled four attacks on the town of Rann since December, according to the report titled Displacement to Rann Town. 

All those who fled were ordered to return to northeast Nigeria, the aid agency Medecins Sans Frontieres said earlier in the day.

"Reports from sources on the ground indicate that hundreds more are en route to Rann from Cameroon (eight kilometers) by foot," the UN report said. "All these people are in dire need of humanitarian assistance, including shelter, food and safe water."

Earlier this month, Boko Haram killed at least 60 people in Rann after it was abandoned by the military, according to Amnesty International and security sources.

That attack came two weeks after the militant group overran the town, driving out Nigerian soldiers and signaling the group's re-emergence as a force capable of capturing army bases.

Last month Cameroon was criticized by the UN refugee agency for sending 9,000 people back to Rann and refusing them asylum, breaching the international principle of "non-refoulement" of refugees.

The return to Rann came on the day when Nigeria's President Muhammadu Buhari was declared to have won a second term, but his main rival planned a fraud challenge after a vote marred by delays and violence.

Buhari had promised to improve security, having made a similar promise in his first presidential election campaign in 2015.