ABUJA (AFP) - Boko Haram remains a threat despite "impressive" military gains against it, French President Francois Hollande said on Saturday, as regional and Western leaders gathered for talks on the Islamist threat.

"The results (of the counter-insurgency) are impressive" and the rebels had been "diminished and forced to retreat", he told a news conference in Abuja.

But added: "This terrorist group nevertheless remains a threat."

Hollande was speaking after talks with his Nigerian counterpart Muhammadu Buhari at the presidential villa in the Nigerian capital before regional leaders met to discuss the conflict.

Nearly seven years of violence have left at least 20,000 dead and left more than 2.6 million homeless, devastating infrastructure in Nigeria's remote northeast and creating a humanitarian crisis.

Discussions at the security summit are expected to focus on the formal deployment of a new regional force comprising troops from Nigeria and its neighbours Benin, Cameroon, Chad and Niger.

Over the last 15 months, individual armies have largely been acting independently to curb the violence in the face of mounting cross-border attacks, particularly suicide bombings against civilians.

The UN Security Council has also raised concerns about Boko Haram's links to the Islamic State group, after reports of Nigerian fighters in lawless Libya.

Buhari said progress had been made, as Boko Haram, which captured swathes of territory in 2014 and declared a self-styled caliphate, was "now... not holding" any local government districts in the north-east.

But he said the "main problem now is rehabilitation" of destroyed infrastucture such as schools, health clinics, roads and bridges, as well as handling the displaced, more than 60 percent of whom are women and children, many of them orphans.

"This is a pathetic situation and is a major problem we are going to face in this country."

Hollande, who sees France as a natural liaison between its former colonies and English-speaking Nigeria, said results had been achieved through better regional coordination.

Paris had also provided training and equipment, he added, as part of the international support to Abuja that includes British military trainers and US surveillance drones.

Hollande and Buhari signed a "letter of intent" to pave the way for a defence agreement.

The two leaders also signed several accords strengthening existing cooperation, including through France's main development agency, of upwards of US$120 million (S$165 million) for Nigeria's under-capacity electricity sector.