Behind the ISIS group that ambushed US forces in Niger The group, ISIS in the Greater Sahara, pledged allegiance to ISIS in 2015.

 -- The Defense Intelligence Agency assesses it is "highly likely" that the group behind the ambush in Niger that killed four U.S. service members was ISIS in the Greater Sahara, a U.S. official confirmed to ABC News.

Other terror groups operate in that region, but none have claimed the Oct. 4 attack, the official said.

The recent death of four U.S. troops in Niger has highlighted the American military presence in West Africa.

U.S. Army Green Berets are in Niger as part of a counterterrorism mission to train that country's military to help fight Islamic extremist groups, including ISIS in the Greater Sahara, in neighboring countries like Mali.

What is ISIS in the Greater Sahara?

ISIS in the Greater Sahara (ISGS) was established in 2015 after the group's current leader, Adnan Abu Walid al-Sahrawi, broke from an al-Qaeda group and pledged allegiance to ISIS and its leader, Abu Bakr al-Baghdadi.

According to the Pentagon, ISIS leaders in Syria have acknowledged al-Sahrawi's allegiance through their Amaq news agency, but ISGS "has not been formally recognized as an official branch of ISIS."

The group's first confirmed terror attack occurred in September of last year when fighters targeted a customs post in Burkina Faso.

Since then, the group has continued to carry out attacks against regional security forces in Burkina Faso and Niger, as well as in Mali, where ISGS targets pro-government militias that support the French and United Nations forces in that area.

The Pentagon said ISGS typically uses "small arms and mortars to conduct ambushes and complex attacks."

What other terrorist groups operate in the area?

Since 2015, the al-Qaeda-aligned Jamaat Nusrat al-Islam wal Muslimeen (JNIM) has conducted attacks killing Westerners at hotels in Bamako, Mali, Ougadougou, Burkina Faso, and Grand Bassam, Cote d'Ivoire.

The Pentagon said there is a high risk of kidnapping in the region due to the operation of extremists groups like ISGS and JNIM.

In October 2016, JNIM abducted an American aid worker from his home in Abalak, Niger. He is one of six hostages believed to be currently held by JNIM.

The others are from Australia, Romania, Switzerland, Colombia, and France. All were abducted in Burkina Faso, Niger or Mali.

"JNIM recently released two Western hostages held for over five years, and received multi-million dollar ransoms for each," the Pentagon said.

Another terror group in the region is Boko Haram, a pledged ISIS affiliate, which operates in Nigeria, Chad, Cameroon and Niger.

ABC News' Luis Martinez contributed to this report.