By Madjiasra Nako and Abdoulaye Massalaki
N'DJAMENA/NIAMEY (Reuters) - Chad and Niger launched a joint army operation against Boko Haram militants in Nigeria on Sunday, military sources said, intensifying a regional push to try to defeat the Sunni Islamic group that has killed thousands in a six-year insurgency.
Boko Haram has expanded cross-border raids into Cameroon, Chad and Niger in recent months, spurring Nigeria's neighbors to retaliate, although cooperation between them and the government in Abuja has been limited and at times strained.
Sunday's strike marks Niger's first incursion deep into Nigerian territory; the country's troops had until now only fought Boko Haram in the border area.
Chad has already sent troops many kilometers inside northeastern Nigeria, winning back areas from the Sunni jihadist group near the Nigeria-Cameroon border.
"We can confirm that Chadian and Nigerien forces launched an offensive this morning from Niger. The offensive is underway," said Colonel Azem Bermandoa, spokesman for Chad's army.
A witness in Niger's southeastern town of Diffa said he saw a convoy of at least 300 vehicles, including army jeeps equipped with heavy weapons as well as water and fuel trucks, leaving barracks overnight and heading to the border.
"I was told their plan was to join forces with other Niger troops from Bosso," he said referring to another border town.
Niger military sources said troops were attacking militants from the Islamist group in Nigeria's Borno State, without giving details of the sites. One of the sources said that Abuja had given the green light for the operation, although this could not immediately be confirmed with Nigerian authorities.
The African Union endorsed a plan to set up a regional force to combat the group in late January and is pushing for a U.N. Security Council mandate for the operation.
It was unclear how many soldiers were involved in Sunday's military push, which came the day after suspected Boko Haram insurgents killed at least 50 people in four bomb attacks in Maiduguri, in northeastern Nigeria.
The blasts were the worst attacks in the city, Boko Haram's birthplace and the chief town of Borno state, since the group tried to take it in late January and again in early February.
Boko Haram wants Maiduguri as the capital of the Islamist state it wants to create in northeastern Nigeria. It currently controls territory in that region about the size of Belgium.
In a symbolic move on Saturday, the group pledged allegiance to Islamic State, the violent jihadi organization that rules a self-declared caliphate in parts of Iraq and Syria.
The pledge led Nigeria's government to appeal for greater international help in combatting the insurgency, which Nigeria's electoral commission cited as the reason for its six week postponement of presidential elections to March 28.
(Writing by Emma Farge and Jon Boyle; Editing by Sophie Walker)