Three soldiers have been killed in intense fighting between Nigeria's army and jihadists over a key northeastern garrison town, a security source said Wednesday.The soldiers died when an army vehicle burst into flames after a car filled with explosives rammed into a convoy on Tuesday afternoon, the source told AFP.The attack occurred during fighting with Islamic State West Africa (ISWAP) militants in the town of Monguno some 135km north of Borno State capital Maiduguri.Just three days earlier, Chad withdrew 1,200 troops from the region - including hundreds from Monguno - after completing a nine-month mission fighting a rival jihadist group, Nigeria's Boko Haram. The pullout from the largest base in the volatile Lake Chad region, which straddles the borders of Nigeria, Niger, Cameroon and Chad, had sparked widespread fears of increased jihadist attacks.On Tuesday, dozens of ISWAP fighters aboard several trucks fitted with machine guns attacked Monguno.Residents in Monguno said a rocket-propelled grenade struck a refugee camp housing thousands of people, causing a fire that gutted hundreds of tents."We still don't know if anyone was killed in the camp but some people sustained burns," said Monguno resident Kulo Gana.Residents also said jihadists ambushed buses carrying civilians under military escort outside the town, sparking a gun battle.Soldiers who repelled the attack killed many of the jihadists, witnesses said. Militants also stormed a compound housing the offices of aid agencies working in the region.Residents said they launched grenades into the offices, which had been evacuated before the attack.Soldiers quelled the attacks and put the town on lockdown on Wednesday, with troops stepping up patrols, residents said.ISWAP and Boko Haram have repeatedly attacked Monguno in failed attempts to overrun it.In recent months, ISWAP has ramped up attacks in the northeast, targeting security officials and abducting civilians at fake highway checkpoints. The decade-long conflict in remote northeastern Nigeria has claimed 36,000 lives and displaced around two million from their homes, according to the United Nations.The conflict has spilled into neighbouring Niger, Chad and Cameroon, prompting a regional military coalition to fight the jihadist groups.