A huge explosion is seen at a military munitions depot in the city of Zarqa, east of Amman. AFP

A series of massive explosions that rocked Jordan's second largest city early on Friday was caused by mortar shells stored at an army munitions depot warping in an intense heat wave, the army said.

The government had earlier blamed an electric short circuit for the dawn blasts, which lit the desert sky and could be seen in the capital Amman, 35 km (22 miles) to the southwest. Both it and the army command said there were no reports of injuries.

Army spokesman Brigadier General Talal Al Ghobain said investigations now indicated the intense heat caused the "thermal expansion of mortar shells" in the arms depot on the eastern outskirts of the sprawling city of 1.5 million people.

The blast site lies within a high security zone where some of the country's major US-equipped army bases are located, military sources say.

A member of Jordanian security forces stops cars east of Amman. AFP

"We felt like an earthquake had struck. Our windows shook and glass shattered. My kids started crying," said Zarqa resident Nabila Issa, a housewife and mother of five children.

Government spokesman Amjad Adailah said earlier that mortars stored at the facility were old and not usable. An army source said on condition of anonymity that some of the weapons at the site were precision-guided anti-aircraft missiles; an army spokesman said no such missiles were stored there.

Temperatures in the kingdom, along with neighbours Syria, Israel, Palestinian territories and Iraq, have topped 45 degrees Celsius (113°F) this month, levels not seen in decades.

Agence France-Presse