An explosion ripped through several floors of a block of flats in Bucharest on Friday, killing at least three people and putting 13 in hospital, Romanian rescue services said, saying the cause was probably gas.

The explosion severely damaged two floors of the eight-storey block, putting the building at risk of collapse and forcing the evacuations of residents, as well as pupils and teachers in a nearby high school in the southwest of the Romanian capital.

Rescue services were still searching for possible victims, with one found trapped under rubble on the sixth floor, they said.

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The most likely cause of the explosion is gas, said rescue services head Raed Arafat, without giving further details.

“The building is at risk of collapse, which is why it is being completely evacuated by firefighters,” he told television stations at the scene.

He added the authorities decided to close off the area, deeming it dangerous.

“I thought we were being bombed,” one witness told local media, while others said they thought an earthquake was happening.

Two of the people hospitalised are in serious condition, according to authorities.

The explosion also damaged windows at the nearby school that was evacuated.

Some pupils were injured, the Bucharest school inspectorate said, without saying how many.

“The pupils who suffered minor injuries received medical attention at the school and were subsequently picked up by their parents,” it said in a statement.