At least two people were killed, and several others remain missing after a "catastrophic" explosion partially destroyed a nursing home in eastern Pennsylvania, the state’s governor said.

Authorities in Bucks County, north of Philadelphia, said a gas leak may have caused the explosion at the Silver Lake Nursing Home, triggering a partial collapse and trapping residents inside the burning structure.

"At this time, there are at least two fatalities. We know that there are some number of individuals still missing," Governor Josh Shapiro told a news conference.

Shapiro praised first responders for their swift action, saying firefighters were "quite literally climbing up ladders, handing individuals in this nursing home to police," with officers at times carrying two people at once to safety.

Bristol Township Fire Chief Kevin Dippolito said crews detected "a heavy odour of gas" when they arrived at the scene.

"When the explosion occurred, there was a major structural collapse, with parts of the first floor into the basement, with persons trapped," Dippolito said.

He added that firefighters rescued residents from windows, doors, stairwells and elevator shafts.

Dippolito said five people remained unaccounted for, stressing that the figure was preliminary.

"We are still in rescue mode until we know the building is clear," he said.

Shapiro said the nursing home had new owners as of this month, adding that state health inspectors visited the facility on December 10 and met with staff to develop a plan to update operational standards.