Washington - After an extensive search, a US soldier who had been missing for nearly two days in Niger has been found dead, a result of a deadly ambush by dozens of Islamic extremists on a joint patrol of American and Niger forces, US officials said on Friday.The soldier, whose name has not been released, was one of four US troops and four Niger forces killed in the attack.His body was found by Niger forces on Friday near where the ambush occurred, and then transferred into US custody at a safer location further from the attack site. US officials said his body was moved onto an American helicopter by US forces in a somber ceremony and then taken away for formal identification.Eight Niger soldiers and two US troops were wounded in the attack, but they were evacuated from the area on Wednesday after the attack unfolded.US officials described a chaotic assault, as 40-50 extremists in vehicles and on motorcycles fired rocket-propelled grenades and heavy machine guns at the patrol, setting off explosions and shattering windows. The soldiers got out of their trucks, returning fire and calling in support from French helicopters and fighter jets that quickly responded to the scene, according to officials. The officials weren't authorised to discuss the matter publicly so spoke on condition of anonymity.US officials say they believe extremists linked to the Islamic State group were responsible for the attack about 200km north of Niger's capital, Niamey.The US and Niger forces were leaving a meeting with tribal leaders when they were ambushed. Most of the US troops were Army special forces.According to a statement by Niger's army chief of staff, the joint patrol was attacked by "terrorist elements" in a dozen vehicles and about 20 motorcycles.The statement said the deaths and injuries came "after intense fighting, during which elements of the joint force showed exemplary courage."US special operations forces have been routinely working with Niger's forces, helping them to improve their abilities to fight extremists in the region. That effort has increased in recent years, the Pentagon said.Mali, Mauritania, Niger, Burkina Faso and Chad are putting together a 5 000-strong G5 Sahel force to fight the growing threat from extremists in the vast Sahel region. The first units are expected to deploy in October and all battalions should be on the ground by March 2018.The Security Council unanimously adopted a resolution in June welcoming the deployment, but at US insistence it did not include any possibility of UN financing for the force.That force will operate in the region along with a 12 000-strong UN peacekeeping mission in Mali, which has become the deadliest in the world for UN peacekeepers, and France's 5 000-strong Barkhane military operation, its largest overseas mission.