A gas explosion at a highway construction site in Romania early Thursday killed four people and injured five, emergency authorities said.

The blast happened around 1 am local time near the eastern town of Calimanesti, said the emergency office in a statement.

It was caused by the “cracking of a gas transport main and appearance of mechanical sparks” during construction work, spokesperson Florin Olaru said.

Local media said workers hit the pipeline with a digger, causing the explosion.

Authorities sealed off the area some 500 metres from the blast site as fire fighters worked to contain the fire, the statement added.

Four people were found dead on site while five with burns were transferred to an emergency hospital in the town of Adjud.