Kano - Four west African states have launched a major offensive against the Boko Haram jihadist group in Nigeria, the military said on Tuesday.Soldiers from Nigeria and the neighbouring countries of Cameroon, Chad and Niger are targeting a Boko Haram faction led by Abubakar Shekau in the Sambisa Forest, and another led by Mamman Nur, on and around Lake Chad.Both locations are in Borno state, northeast Nigeria.According to the Nigerian military, scores of jihadists have been killed and hundreds of others have been forced to surrender in recent days.Top brass in Abuja have claimed that Nur had been injured and one of his wives killed in an aerial bombardment.Shekau was "a spent horse, waiting for his Waterloo," army spokesman Brigadier General Sani Usman said on Monday.On Tuesday, he said the operation - codenamed "Deep Punch 2" - had been making "tremendous progress".But he said four soldiers had "paid the supreme price" and nine others were wounded by a suicide car bomb attack against a military vehicle near Shekau's camp on Monday.Separate senior military and civilian militia sources spoken to by AFP gave a higher death toll of 10.Two Cameroon security sources meanwhile said two of its troops had been killed in the Sambisa Forest, although it was not clear if they were among those mentioned by Usman.Repeated attacks Boko Haram has been fighting to establish a hardline Islamic state in northeast Nigeria since 2009. The violence has killed at least 20 000 and displaced more than 2.6 million.In 2015, the four neighbours joined forces with Nigeria's tiny western neighbour, Benin, to set up a Multi-National Joint Task Force, designed seize back territory lost to the jihadists.Nigeria's president, Muhammadu Buhari, a former military general who is expected to seek a second term next year, was elected in 2015 on a promise to defeat the insurgents.He and military commanders have repeatedly insisted the Islamic State group affiliate is a spent force but regular attacks still occur in the remote region. The Nur faction, which goes by the name Islamic State West Africa Province (ISWAP), last Friday claimed a late December raid on a Nigerian military base that killed nine soldiers.Shekau appeared in a new video message last Tuesday and claimed a series of recent attacks in the Borno state capital, Maiduguri and the border towns of Gamboru and Damboa.Heavily fortified Shekau, Boko Haram's leader since 2009, pledged allegiance to ISIS in 2015, just as the military counter-insurgency began to make gains.But he was enraged in August 2016 when IS gave its backing to the faction led by Abu Mus'ab Al-Barnawi, the son of Boko Haram founder Mohammed Yusuf.Nur - Shekau's former right-hand man and the mastermind of the 2011 UN office bombing in Abuja - is seen as the faction's de-facto leader and Al-Barnawi the figurehead.The operations against Shekau are concentrated on his hideout in the Parisu area of Borno, according to military and civilian militia sources.One militia leader said the suicide car bomb attack that killed the troops happened at Lagara village, where soldiers had been forced to stop because of Boko Haram landmines."There are mines all over the marshland leading to the Shekau camp, which makes it difficult for troops to get to the camp," he said."Once the soldiers cross the river, they are in Parisu. Shekau is within their grasp this time because he is holed up."Previous escape Shekau, whom the authorities have claimed to have killed on a number of occasions, has been surrounded and managed to escape before. In December 2016, he evaded troops who overran his Camp Zairo enclave in the Sambisa Forest.The military claims to have liberated the former game reserve but Boko Haram is now said to be back in the camp.Nur, who is believed to have links with al-Qaeda affiliates in north Africa, was said to have been injured in an air raid on a militant camp on Tumbin Kare island, on Lake Chad.He had moved there from his headquarters in Tumbin Gini last week, according to the militia leader."Mamman Nur never stayed in one particular location. He moved around the islands under his control to avoid detection," he added. "He was not lucky this time."Security agencies have warned the public about Barnawi/Nur fighters "attempting to melt into other communities" in the northern states of Kano, Yobe and Jigawa."The concern is that the terrorists would take cover among the civilian population to wreak havoc," they said.