French President Francois Hollande told reporters after meeting his Nigerian counterpart Muhammadu Buhari in Abuja that "impressive" gains had been made against the Islamists by greater cooperation.
But he warned: "This terrorist group nevertheless remains a threat."
Buhari has invited leaders from Benin, Cameroon, Chad and Niger, whose troops will make up a new regional force against Boko Haram, which has been pushed to northeast Nigeria's borders around Lake Chad.
Plugging gaps and improving coordination between armies that are currently operating largely independently is seen as vital in the remote region where borders are notoriously porous.
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The waters of Lake Chad form the border between Nigeria, Cameroon, Chad and Niger, which have all been attacked by the militants in raids and suicide bombings.
Nigeria is seeking closer military cooperation to bring to an end nearly seven years of violence, which has left at least 20,000 dead and displaced more than 2.6 million people in the northeast.
But it also expressed "deep concern" at Boko Haram's threat to security in West and Central Africa and "alarm at... linkages with the Islamic State", which operates in Syria, Iraq and Libya.
Boko Haram's shadowy leader Abubakar Shekau pledged allegiance to his IS counterpart Abu Bakr al-Baghdadi last year, although there has since been little evidence so far of direct support on the ground.