Hundreds of Chadian ground troops entered northeastern Nigeria Tuesday as part of a growing regional offensive against Boko Haram insurgents.
Witnesses said Chadian soldiers and vehicles crossed a bridge from Cameroon into the Nigerian town of Gambaru. Another Chadian force of at least 300 soldiers positioned itself near Bosso, a town on the border between Nigeria and Niger.
Nigerian government spokesman Mike Omeri told VOA's Hausa news service that Chad is operating under a bilateral protocol that allows them "to pursue terrorists into this territory."
Chad is one of four countries that have pledged to help fight Boko Haram, along with Cameroon, Niger and Benin. The Islamist insurgents control parts of Nigeria's Borno state and seized a multinational base on the shores of Lake Chad last month.
Some towns retaken
The Nigerian government said late Monday its forces have retaken a number of towns from Boko Haram, including Gambaru, Mafa, Mallam Fatori, Abadam and Marte.
But the French news agency AFP quoted a Chadian army officer as saying Boko Haram fighters remain in Gambaru and "they have posted snipers everywhere."
Northeastern Nigeria has seen several days of heavy fighting, with both the Chadian and Nigerian air forces targeting the militants.
Meanwhile, police said a bomb blast that went off just minutes after Nigeria's President Goodluck Jonathan left an election campaign rally was the work of a female suicide bomber.
The bomb exploded Monday near a stadium in the northeastern Nigerian city of Gombe, setting a car ablaze, killing one person and wounding 18.
Gombe state police spokesman Fwaje Atajiri said by telephone that the bomber had been a female with explosives strapped to her, a sign it was the work of Islamist militant group Boko Haram, although no one has claimed responsibility.
Nigeria is due to hold a presidential election February 14, pitting the ruling People's Democratic Party's (PDP) Jonathan against former military ruler Muhammadu Buhari for the opposition All Progressives Congress (APC).
Boko Haram militants are stepping up their campaign of violence ahead of the election.
Attack on Maiduguri
On Sunday, a combined force of Nigerian soldiers and civilian fighters drove back the militants' assault on Borno state capital, Maiduguri. The attack was Boko Haram's second unsuccessful attempt to seize the city in a week.
In a travel warning Tuesday, the U.S. warned Americans to avoid all travel to the states of Adamawa, Borno and Yobe -- the three states where Boko Haram is most active.
All three states are under a Nigerian government-imposed state of emergency.
The Islamist insurgents are blamed for thousands of deaths since 2009.