A black-clad gunman killed two children and wounded 17 other people on Wednesday, when he opened fire on students attending Mass at a Minneapolis Catholic school, authorities said, a shocking outburst of violence at the beginning of the US school year.

The assailant, a man in his early 20s, fired dozens of rounds through the church windows at students sitting in church pews and then took his own life, officials said. The children killed were 8 years old and 10 years old, they said.

"This was a deliberate act of violence against innocent children and other people worshiping. The sheer cruelty and cowardice of firing into a church full of children is absolutely incomprehensible," Minneapolis Police Chief Brian O'Hara told reporters.

The shooting occurred two days after school started at Annunciation Catholic School, a private elementary school with about 395 students connected to a Roman Catholic church in a residential area in the southeast part of Minnesota's largest city. Local TV showed parents ducking under yellow police crime tape and leading students out of the school.

At least two of the chapel doors had been barricaded from the outside using 2" x 4" planks, O'Hara said.

Officials said the shooter was armed with a rifle, a shotgun and a pistol and did not have an extensive criminal history.

They did not provide his name and said they were trying to identify a motive. Officials also said they found a smoke bomb at the scene and were searching a vehicle in the parking lot.

Law enforcement was investigating multiple online videos to determine if they were posted by the shooter, according to two sources who spoke on condition of anonymity.

Local hospitals said they were treating 15 children and two adults, with many suffering gunshot wounds.

School shootings occur on a regular basis in the US, spurring ongoing debates about gun laws and school safety.

There have been more than 140 such incidents so far this year, according to the K-12 School Shooting Database.

"Don't just say this is about thoughts and prayers right now. These kids were literally praying. It was the first week of school. They were in a church," Minneapolis Mayor Jacob Frey said at a news conference, visibly angry.

US President Donald Trump said he had been briefed on the shooting and said the FBI was on the scene. "Please join me in praying for everyone involved!" he said on social media.

There have been three other shootings in the midwestern city since Tuesday afternoon, including one at a Jesuit high school, that have together left three people dead and seven wounded, according to police.

Wednesday's shooting did not appear to be related to the others, O'Hara said.

Minneapolis has experienced a significant rise in homicides in the years following the 2020 police killing of George Floyd, which prompted nationwide protests, civil disturbances and staffing shortages in the city's police department.

The city recorded 54 homicides last year, down from 71 in 2021 but well above the 29 recorded in 2019.

Minnesota as a whole has a gun death rate below the national average, according to Everytown for Gun Safety, a gun violence prevention group.

Minnesota also experienced an outburst of political violence in May, when a gunman posing as a police officer killed the Democratic state House speaker and her husband and wounded a Democratic state senator and his wife, in what authorities said were targeted assassinations. The suspect has pleaded not guilty to federal murder charges.