A natural gas explosion on Wednesday (2 August) at Minnehaha Academy's upper school in Minneapolis, Minnesota killed one person, left one unaccounted for and injured at least nine others, including one critically.

The late morning blast caused a partial building collapse and set the building ablaze. Footage of the scene posted on social media shows smoke billowing from the debris.

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Fire Chief John Fruetel confirmed a body was pulled from the rubble just before a 2.45pm local time.

"Of the two missing, we have located one; just one," Fruetel said, according to the Star Tribune.

Minnehaha Academy confirmed the fatality and identified the victim as receptionist Ruth Berg, who worked with the academy for 17 years. "As our receptionist, she welcomed everyone with a smile and was always willing to go the extra mile to help our students, families, and staff," the school said in a Facebook post.

"She will be greatly missed. Please keep Ruth's family, and our school community, in your prayers," the statement added. "Please also keep in your prayers a staff member who is unaccounted for, those who were injured and are currently receiving medical care, and family members."

According to the Star Tribune, a janitor at the school remains unaccounted for. School officials confirmed that 81-year-old John Carlson, an alumnus of the school who returned to work as a janitor in 2003, was missing.

Carlson's wife, Barbara Carlson, told the Star Tribune: "I'm not doing well. I haven't heard from my husband, and I'd like to keep the line free."

Fruetel said a search is underway to find Carlson and it is being considered a rescue operation.

"It's just a very precarious situation, it's very, very unstable, there are no signs of (the missing person's) obvious location and we know there are probably at least two floors of debris," Fruetel said. "We still consider it a rescue at this point; we don't really know we're going to do our best to locate everyone that's unaccounted for."

Year-round staff, girls participating in a summer cross-country practice, as well as basketball and football players were all on campus during the 10.20am blast.

Of the nine people who were injured, Hennepin County Medical Center officials said four remained hospitalised. One patient was in critical condition and three more were in a satisfactory condition.