A US soldier pleaded guilty Monday to conspiring with his cousin to attack a suburban Chicago military base with the aim of killing over 100 people, prosecutors said.

Hasan Edmonds, 23, was a soldier in the US Army National Guard when he began communicating online in January with an FBI agent pretending to be an Islamic State fighter in Libya.

Edmonds admitted to giving the purported militant advice on "how to fight and defeat the US military," the plea agreement said. He said he and his cousin were ready to conduct an attack in the United States.

Edmonds and his cousin began meeting with another undercover agent in March who pretended to help Edmonds plan a trip to Mosul, Iraq to join the Islamic State group.

They also discussed a plan for Jonas Edmonds, 30, to use his cousin's military uniforms to attack the base in Joliet, Illinois where Hasan Edmonds was assigned.

Jonas said he wanted the undercover agent to help him achieve a "body count" of 100 to 150 people.

Hasan Edmonds took his cousin and the undercover agent to the base on March 24 to conduct surveillance and retrieve a training schedule in order to pick the best time to attack, the plea agreement said.

Jonas Edmonds drove his cousin to the airport the next day, but Hasan Edmonds was arrested before he could board the flight to Egypt. Jonas Edmonds drove from the airport to his cousin's house but was arrested before he could retrieve the uniforms.

"Thanks to the efforts of many prosecutors, agents and analysts, we were able to ensure these plotters did not attain their violent endgames, and with these guilty pleas, they will be held accountable," John Carlin, Assistant Attorney General for National Security, said in a statement.

"Counterterrorism remains the (Justice) Department's highest priority, and we will continue (to) use all available tools to combat ISIL, a foreign terrorist organization that rapes, murders and enslaves Muslims and non-Muslims alike."

Hasan Edmonds faces up to 30 years in prison after pleading guilty to one count of conspiring to provide support to a foreign terrorist organization and one count of attempting to provide material support to a foreign terrorist organization.

Jonas Edmonds faces up to 23 years in prison after pleading guilty last week to one count of conspiring to provide material support to a foreign terrorist organization and one count of lying to a law enforcement officer.

Both men are US citizens.