A 21-year-old man from Michigan, who is believed to be a supporter of the Islamic State (ISIS), has been arrested by federal authorities after telling an undercover FBI agent that he wanted "to shoot up a church one day."

Authorities did not say which church was targeted by Khalil Abu-Rayyan of Dearborn Heights, but the suspect said the church he was planning to attack had a capacity of 6,000 people.

"I don't know the name of it, but it's close to my job. It's one of the biggest ones in Detroit. Ya, I had it planned out. I bought a bunch of bullets. I practised a lot with it. I practised reloading and unloading. But my dad searched my car one day, and he found everything. He found the gun and the bullets and a mask I was going to wear," Abu-Rayyan said, according to MSNBC and The Detroit News.

The FBI had been investigating Abu-Rayyan since May "regarding increasingly violent threats he has made to others about committing acts of terror and martyrdom — including brutal acts against police officers, churchgoers and others — on behalf of the foreign terrorist organisation Islamic State of Iraq and Levant."

He allegedly used Twitter to express his support for ISIS.

The suspect expressed frustration for not being able to carry out his plan to stage a massacre inside the church.

"Honestly I regret not doing it. If I can't do jihad in the Middle East, I would do my jihad over here," he said.

He also told the FBI agent that, "It is my dream to behead someone."

Despite his incriminating statements, Abu-Rayyan is not facing terrorism charges as of this time but only gun and drug charges stemming from an October 2015 speeding incident with a pistol, sleeping pills and marijuana, according to WDIV. Terror charges could be brought up later.

He was scheduled to appear in court Monday for a detention hearing.