Chad hunts attackers after 40 killed in Boko Haram raid

Around 20 people were also wounded, military sources said.

Chadian President Mahamat Idriss Deby Itno visited the scene early on Monday and launched an operation "to go after the attackers and track them down in their furthest hideouts", the statement added.

The GCAP opposition platform, which is composed of 15 fiercely anti-government parties, demanded that the circumstances in which the onslaught took place be clarified, fearing a possible "security breach".

"Given that the area has been declared a military zone, and given the advanced state of belligerence, our soldiers did not get ambushed while on a tactical mission. If they were indeed on a military operation with this many casualties, a thorough clarification is expected," GCAP spokesman Max Kemkoye posted on Facebook.

The garrison's unit commander was among the dead, a high-ranking officer said on condition of anonymity.

The assailants "had time to seize ammunition and equipment before they retreated", the officer added.

"Boko Haram members took control of the garrison, seized the weapons, burnt vehicles equipped with heavy arms, and left," said one local source, who asked not to be named.

The surprise attack struck a Chadian army position near the Nigerian border late on Sunday evening, military sources said.

"We have many casualties, yes, but the situation is under control and our forces are on the ground pursuing the enemy," the regional governor, General Saleh Haggar Tidjani, told AFP.

Frequent attacks

In a vast expanse of water and swamps, the Lake Chad region's countless islets serve as hideouts for jihadist groups, such as Boko Haram and its offshoot Islamic State in West Africa (ISWAP), who make regular attacks on the country's army and civilians.

Boko Haram launched an insurgency in Nigeria in 2009, leaving more than 40,000 people dead and displacing two million, and the organisation has since spread to neighbouring countries.