Tampa - The Islamic extremists that ambushed and killed US Army commando in Niger last week hadn't operated in that area before, Defence Secretary Jim Mattis said on Wednesday, referring to what officials believe was a relatively new offshoot of the Islamic State group there.Speaking to reporters travelling with him to Tampa, Mattis said he rejected suggestions that rescue forces were slow to respond to the assault, noting that French aircraft were overhead within 30 minutes. But he said the US military is reviewing whether changes should be made to these types of training missions in Africa."We will look at this and say was there something we have to adapt to now, should we have been in a better stance," said Mattis. "We need to always look at this. We're not complacent, we're going to be better."US Africa Command has launched an investigation into the attack that will review what went wrong and whether additional security or overhead armed support may be needed for some of these missions.American officials have said they believe the militants belonged to a tribal group that previously may have been tied to al-Qaeda or other extremists, but more recently re-branded themselves as ISIS. The officials said they do not believe the militants were fighters who came to Niger from outside the region. The officials were not authorised to discuss the matter publicly so spoke on condition of anonymity.Three Army commandos and a soldier were killed a week ago when dozens of militants ambushed them during a joint patrol with Niger troops. The US and Niger troops were in unarmoured trucks.Mattis and other officials haven't said how long it took to evacuate the troops, including several US and Niger forces who were wounded. One US Army soldier was missing for nearly two days before he was finally found by Niger troops around the area where the attack happened.According to US officials, details about the exact timeline for the rescue effort are still unfolding. The troops were evacuated by French aircraft.'Golden hour'Army special forces have been working with Niger troops for some time, and that training effort has been increasing in recent years.They are often working in remote locations well beyond what the US military likes to call the "golden hour".That one-hour standard for medical evacuation was set during the peak war years in Iraq and Afghanistan and was aimed at getting wounded troops out within an hour of their injury, making it more likely they will get the treatment needed to survive.Mattis praised the quick response of the French and Niger support forces."The French pilots were overhead with fast movers with bombs on them ready to help, and helicopters were coming in behind," he said adding that Niger forces with French advisers also responded to the attack, which took place 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. There were about a dozen US troops and a company of Niger forces, for a total of about 40 service members in the joint mission.US officials have described a chaotic assault in a densely wooded area, 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 the French aircraft.