Islamist militants killed at least 10 people and took two women captive in an attack on a convoy in northern Nigeria that targeted Christians and those associated with international aid groups, witnesses said on Friday.

In the Dec. 22 attack, they told Reuters, militants posing as soldiers stopped a convoy of commercial vehicles travelling towards Maiduguri in northeastern Borno state and asked passengers in English to produce identification cards.

The militants separated those who they determined worked for international aid groups, were Christian or worked with the police or Civilian Joint Task Force (CJTF), a coalition of civilian vigilante groups formed to fight Boko Haram jihadists.

It opened fire on people associated with these three groups, killing 10, including four who witnesses said had identification cards associating them with humanitarian groups.

“They asked those aid workers, and others, to move towards the north (side of the road) and started shooting at them sporadically, killing them,” local witness Babagana Kachallah told Reuters.

Kachallah said the militants took two women captive who it believed worked for international aid groups.

Two other witnesses, a family member of one of those shot dead and two sources who work with aid groups in Nigeria, confirmed the attack.

Nigeria announced in September that it would require anyone moving through the northeastern states of Adamawa, Borno and Yobe to carry identification cards to help authorities root out members of Boko Haram and Islamic State.

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

Reuters