A shooter has fired dozens of rounds through a church’s stained-glass windows while schoolchildren were praying at Catholic mass, before turning the weapon on himself.

Two children, aged eight and 10, were killed and 17 others were wounded as bullets were sprayed during the back-to-school service at a Minneapolis Catholic school on Wednesday (local time).

The suspect in his 20s, who was wearing black, was armed with a rifle, a shotgun and a pistol, officials said.

“These kids were literally praying. It was the first week of school. They were in a church,” a visibly angry Minneapolis mayor Jacob Frey said.

Minneapolis Police Chief Brian O’Hara said the two children who died were “sitting in the pews at mass when they were shot and killed”.

Fourteen children, aged between six and 15, were “injured by gunfire”. Three worshippers aged in their 80s were also shot.

“This was a deliberate act of violence against innocent children and other people worshipping,” he said.

“The sheer cruelty and cowardice of firing into a church full of children is absolutely incomprehensible.”

Parents comfort their children outside Annunciation Catholic School. Photo: AAP

The shooting came two days after school started at Annunciation Catholic School, a private primary school with about 390 students. It is connected to a Catholic church in a residential area in the south-east part of Minnesota’s largest city.

Ten-year-old Weston Halsne, who was only two seats away from the windows, said his friend saved his life by lying on top of him.

“My friend, Victor, saved me though, because he laid on top of me, but he got hit,” Halsne told local TV network WCCO.

“My friend got hit in the back, he went to the hospital… I was super scared for him but I think now he’s okay.

“I think I got like gunpowder on my neck.”

Halsne said children hid under the church pews and in the gym.

Local TV showed parents ducking under yellow police crime tape and leading students out of the school.

Officials said the shooter had no extensive criminal history.

Robin Westman has been named by several US media outlets. Photo: New York Post

Several US media outlets named the suspect as Robin Westman, 23, from Minneapolis.

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.

The New York Post said the videos showed Westman’s “deep fascination” with  previous mass shooters, including killer Adam Lanza who was responsible for his “favourite” mass killing at Sandy Hook elementary school.

He also rambled about his own apparent plans to attack school children, detailing his thoughts on trying to target the greatest number.

FBI director Kash Patel said the shooting was being investigated as “an act of domestic terrorism and hate crime targeting Catholics”.

There have been 286 mass shootings in the US so far this year, according to the Gun Violence Archive database.

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.

The US Department of Homeland Security is in touch with local authorities and monitoring the situation, Homeland Security Secretary Kristi Noem said on social media.

There have been three other shootings in the midwestern city since Tuesday afternoon, including one at a Jesuit high school. Together they have 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.

Minnesota state law requires background checks for all gun sales and the state has a gun death rate below the US average, according to gun violence prevention group Everytown for Gun Safety.

-with AAP