Images and videos said to have been taken at the scene were rapidly spread on social media, showing a huge ball of flame rising into the sky, followed by the sound of several explosions.
Witnesses said huge orange flames lit the desert night sky and could be seen as far away as the capital Amman, which lies 35 kilometers (22 miles) to the southwest.
“We felt like an earthquake had struck. Our windows shook and glass shattered. My kids started crying,” Zarqa resident Nabila Issa, a housewife and mother of five children, said.
Security forces sealed off the sprawling industrial city, and prevented traffic from leaving or entering.
Journalists who wanted to travel through Zarqa to the blast site were also prevented from doing so.
Government spokesman Amjad al-Adaileh said in a statement that “the explosion occurred in the early hours of Friday ... in a warehouse containing unusable mortar bombs belonging to the armed forces.”