MAIDUGURI, Nigeria (Reuters) - Islamist militants released three aid workers and other civilians in northeast Nigeria who had been held hostage since late December, a United Nations official said on Thursday.

The people were kidnapped on Dec. 22 by militants posing as soldiers who stopped a convoy of commercial vehicles traveling towards the city of Maiduguri, state capital of the northeastern state of Borno.

Islamist militants have waged an insurgency in northeast Nigeria that has killed 36,000 people since 2009 and left 7.1 million people needing humanitarian assistance. Boko Haram, a group seeking a separate state in northeast Nigeria adhering to a strict interpretation of Islamic laws, began the insurgency.

“I am deeply relieved that some civilians, including three aid workers, who were abducted by non-state armed groups along the Monguno – Maiduguri road on 22 December 2019 have been released yesterday and are now safe,” said Edward Kallon, the U.N. humanitarian coordinator in Nigeria, in a statement.

Kallon said he was concerned about the “increasingly insecure environment that humanitarians are working in”. He said a total of 12 aid workers lost their lives in 2019, more than twice the total in 2018, making it one of the most dangerous years for humanitarian actors in Nigeria.

The U.N. did not state whether the militants behind the abduction were associated with Boko Haram or a faction that broke away in 2016 and pledged allegiance to Islamic State. The group -- Islamic State in West Africa Province (ISWAP) -- has in the last two years been the dominant insurgent group in Nigeria.

ISWAP in December said it executed 11 Christian captives it had previously kidnapped in Borno state.