Two children were killed and 17 other people were wounded at a Minneapolis Catholic school when a shooter dressed in black opened fire on students attending Mass, authorities said.

The assailant fired through the windows at students sitting in pews and entering the church and then killed himself, officials said.

Two students, 8 years old and 10 years old, were pronounced dead at the scene, 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.

Officials identified the attacker as Robin Westman, 23.

The shooting took place at Annunciation Catholic School, a private elementary school with about 395 students.

Officials said the shooter fired dozens of rounds, using a rifle, a shotgun and a pistol, which were all purchased legally.

They said they had recovered additional firearms at other residences connected to Westman. Police vehicles were stationed at a house identified by public records as Westman's residence, several miles south of the school.

At least two of the church doors had been barricaded from the outside using wooden planks, O'Hara said.

Local hospitals said they were treating 14 children and two adults, with many suffering gunshot wounds. All were expected to survive, O'Hara said.

Authorities said the attack did not appear to be related to three other shootings over the past 24 hours in Minneapolis, including one at a Jesuit high school.

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