The idea of an anti-jihadist force has been discussed for years by the 12 countries of the Economic Community of West African States (ECOWAS).

The Sahel, which cuts across Africa separating the continent's north from its sub-Saharan part, has for years been plagued by jihadist insurgencies led by groups linked to Al-Qaeda and the Islamic State.

The groups have been particularly active in three former French colonies -- Burkina Faso, Mali and Niger -- that quit the ECOWAS group in recent years following military coups.

The three countries will not take part in the new force, officials said.

Following a meeting of ECOWAS military chiefs in Freetown on February 28 and March 1, details of the new force to "combat terrorism" were decided, a senior Nigerian military official told AFP.

Several participating countries announced the number of soldiers they would contribute, he said.

The initial goal is to have around 2,000 troops ready to fight "terrorism and insecurity" across west Africa, he added.

During the discussions, military chiefs "painted a worrying picture of the regional security landscape", a source close to the meeting said.

"The discussions focused on the rise of the terrorist threat, the expansion of transnational organised crime, and the persistent challenges related to maritime insecurity," the source said.

The troops of the new force will remain stationed in their countries of origin, with Sierra Leone serving as a logistical base for the group, according to an official Sierra Leonean source.

Officials are still thrashing out the details of how to finance the force, a source close to the talks told AFP.

ECOWAS has not yet issued an official statement.

A West African chief of staff told AFP that although Burkina Faso, Mali and Niger have formed their own alliance, they will be encouraged to cooperate with the ECOWAS force.

The three countries "are in a part of the Sahel that is the epicentre of the fight against the jihadists. They must be included," he said.