In a statement, the government said the killings were discovered on the night of 30 December and condemned the "unacceptable violence".

The Reuters news agency quotes prosecutors as saying that the victims were killed by gunfire, but there was no indication on the possible perpetrators or motive for the attacks.

The authorities have called for calm pending the outcome of the investigation.

"This drama occurs at a time when Burkina Faso has initiated an operation of mobilisation of the whole people for unity of action in the fight against terrorism," the government statement said.

A rights group said the dead were civilians who had been targeted by a civil defence force recently launched by the government to help fight an Islamist insurgency. There has been no confirmation of this.

The West African country is battling an Islamist insurgency has already displaced nearly two million people, and prompted two military coups within a year.