The chief of the Army said Monday that U.S. missions to train, advise and assist foreign troops will increase in coming years following the deaths of four soldiers embedded with forces in Niger last week.

The service is standing up its first formal unit this month that is specifically designed to provide the military aid and has plans for six of the so-called security force assistance brigades, thereby "institutionalizing" what had for years been an ad hoc effort, Gen. Mark Milley said.

"We are training, advising and assisting indigenous armies all over the world and I anticipate and expect that'll increase and not decrease in years to come," Milley told reporters at the annual Association of the U.S. Army conference in Washington, D.C.

Three Green Berets and a fourth soldier were killed Wednesday while accompanying Nigerien forces for a train, advise and assist mission in the West African nation where the Islamic State, al Qaeda, and Boko Haram have been active.

The fourth soldier was missing and found dead after a two-day search by U.S. Africa Command and allies including the French. Niger borders Mali and Nigeria, hotspots for violent extremism, and the U.S. has increased its presence in the region in recent years.

"It is a dangerous mission, TAA missions around the world. It depends on where you are at," Milley said.

The new units, which the service calls SFABs, will increase the Army's ability to conduct the assistance missions, which are often focused on countering terrorist groups. All six are expected to be operational within a year or two, Milley said.

The SFABs will comprise about 500 noncommissioned and senior officers. The Air Force, Navy and Marine Corps also provide training and advise to foreign forces.

Meanwhile, questions remain over what exactly led to the deaths of the four soldiers in Niger. Africa Command said last week it had not anticipated the ambush.

"AFRICOM is reviewing very closely the security procedures that they are using for these teams that are there in Africa," Milley said.

The Islamic State has inspired splinter groups in Niger and is suspected in the attack, but Milley declined to say who was responsible.

"I prefer at this juncture at a public meeting not to address that because I am not 100 percent sure that the information is 100 percent accurate," he said. "We do have information on the group that did it, their nature, their disposition, and so on and so forth. Appropriate organizations within the United States military are digging deeper into that for appropriate action."