NEW YORK — An 18-year-old in Idaho allegedly planned a violent rampage of local churches, after he pledged loyalty to the Islamic State (IS) group.

Alexander Mercurio was arrested Saturday, one day before he intended to attack churches with guns, "flame-covered weapons", explosives, knives, a machete, and a pipe.

He allegedly wanted to move from church to church until being killed, the Justice Department said. He could face up to 20 years in prison.

Mercurio is charged with attempting to provide material support or resources to a foreign terrorist organization.

"The defendant swore an oath of loyalty to ISIS and planned to wage an attack in its name on churches in Coeur d'Alene, Idaho," US Attorney General Merrick Garland said in a statement, using a common name for the Islamic State group.

The young Idahoan allegedly wanted to carry out an act of martyrdom in the city in the north-western part of state, where he lived, before the end of the Muslim holy month of Ramadan.

In messages that Mercurio allegedly sent earlier this year, he outlined a plan to "kill as many as possible" at the nearest church. He then said he would "rinse and repeat" that same attack on neighboring churches, setting them on fire as we he went, until he was killed.

Part of the plan allegedly involved beating and handcuffing his father in order to obtain guns from a locked closet in the family home.

Mercurio was arrested after sharing his intentions with an unnamed FBI source, according to the criminal complaint against him.

Mercurio had been known to the FBI under an online alias since July 2022, the complaint says. His beliefs developed in an online group chat, according to messages included in the court documents.

At first, the teenager expressed his early interest in Islamic fundamentalism and IS which the US considers a terror group. He also spoke of his frustration that his parents did not understand his religious views.

But his messages to the chat soon reflected his intentions to "donate every last cent in my bank" to IS and to carry out an attack in the US if he could not join the group in Africa or the Middle East, investigators allege.

The FBI executed a search warrant at Mercurio's family home on Saturday, seizing a pipe, butane, hand sanitizer, a machete and the firearms locked in his father's closet.

The FBI and the Department of Homeland Security had recently issued a bulletin warning Americans of "possible threats to public gatherings in the United States".

The law enforcement bulletin was issued shortly after an attack on a Moscow concert hall killed more than 100 people, for which IS has claimed responsibility. — BBC