Two children and the shooter were killed while 20 others injured in a gunfire incident that occurred on Wednesday morning at the US's Minneapolis Catholic school, Minnesota Governor Tim Walz said.

The shooter was among those killed, a US Justice Department official told Reuters.

Children were present at a morning mass when the shooting happened, according to reports.

"I'm praying for our kids and teachers whose first week of school was marred by this horrific act of violence," Walz wrote on X.

Police, FBI, and other federal agents rushed to the school, and students were evacuated.

The shooting occurred two days after school started at Annunciation Catholic School, a private elementary school with about 395 students.

The school is connected to Annunciation Catholic Church, and both are located in a residential area in the southeast part of Minnesota's largest city.

In a post on Truth Social, President Donald Trump said he was briefed on the "tragic shooting" and that the White House would continue to monitor the situation.

Established in 1923, the pre-kindergarten through eighth-grade school had an all-school Mass scheduled at 8:15 am on Wednesday, according to its website.

Monday was the first day of school, and social media photos from that day show students in green uniforms greeting each other at bicycle racks, smiling for the camera and sitting together.