Chad's army has often played a key role in the decade-long fight against Boko Haram and more recently against the Islamic State West Africa Province (ISWAP) that operate in a region straddling both countries.

Deby, 37, came to power at the head of a military junta after the shock death last month of his father Idriss Deby Itno, who had led the country for 30 years.

"Nigerians know and appreciate the role Chad played in helping us to combat terrorism, and we will continue the collaboration," Buhari said in a statement from his office, without giving further details.

The 78-year-old former general also offered to assist Chad "to stabilise and return to constitutional order."

Deby died, according to the Chadian authorities, on April 19 after suffering mortal wounds fighting the Front for Change and Concord in Chad (FACT), a large armed group with a rear base in Libya.

The new junta appointed a transitional civilian government and promised to hold elections within 18 months.

"We will help in all ways we can," Buhari said, "to ensure a smooth transition in 18 months, as you have promised your people."

The statement added that Deby "recommitted to democratic, free, fair polls in 18 months" and was ready to be "guided" by Nigeria.