A thunderous explosion at a nursing home just outside Philadelphia has killed at least two people, collapsed part of the building, sent fire shooting out and left people trapped inside, authorities said.

Pennsylvania governor Josh Shapiro said on Tuesday that emergency responders braved the flames, the heavy odour of gas and a second explosion to evacuate residents and employees.

Fire officials said they were in “rescue mode” five hours later, with responders still digging by hand and using search dogs, earth-moving equipment and sonar to locate potential victims.

The explosion happened at Bristol Health & Rehab Center in Bristol Township, just as a utility crew had been on site looking for a gas leak. Shapiro said a finding that a gas leak caused the explosion was preliminary.

A plume of smoke rose from the nursing home as emergency responders from across the region rushed there.

Authorities did not identify those who died and did not immediately know the total number of people injured.

The town’s fire chief, Kevin Dippolito, said five people were still unaccounted for but cautioned that some might have left the scene with family members.

Shapiro asked his fellow Pennsylvanians to take a moment to pray “for this community, for those who are still missing, for those who are injured, and for those families who are about to celebrate Christmas with an empty chair at their table.”

Dippolito described a chaotic rescue where firefighters found people stuck in stairwells and elevator shafts and pulled residents out of the fiery building through windows and doors. Two people were rescued from a collapsed section of building, he said.

Firefighters handed off patients to waiting police officers outside, including one “who literally threw two people over his shoulders,” Dippolito said. “It was nothing short of extraordinary.” A second explosion erupted during the rescue, he said. (AFP)