WASHINGTON (REUTERS) - A US Army National Guard soldier and his cousin have been arrested on charges of conspiring to support Islamic State in Iraq and Syria (ISIS), the Department of Justice said on Thursday.

The two Chicago area men, both US citizens, spoke of using army uniforms, along with military knowledge and access, to attack an Illinois military installation, the department said in a statement.

Army National Guard Specialist Hasan Edmonds, 22, was arrested at Chicago Midway International Airport while attempting to fly to Cairo, Egypt, the Justice Department said.

Jonas Edmonds, 29, was arrested at his Aurora, Illinois, home.

Both defendants met with an undercover Federal Bureau of Investigation employee and presented a plan to carry out an armed attack against a northern Illinois military facility where Hasan Edmonds had been training, according to the complaint.

"Disturbingly, one of the defendants currently wears the same uniform of those they allegedly planned to attack," said Assistant Attorney-General for National Security John Carlin, in a statement.

If convicted, they face a maximum penalty of 15 years in prison and a US$250,000 (S$340,000) fine.