At least 18 people have been killed in a lorry blast in the southern Pakistani city of Hyderabad.

But police said Monday's explosion was most likely an accident.

Sources told Al Jazeera a gas cylinder at a nearby shop exploded, which in turn triggered an explosion in the lorry in the Hala Naka area.

Mohammad Ali Baloch, a senior police official, said at least 40 people had been injured when the lorry containing chemicals that was parked near a scrapyard, exploded.

The lorry was parked at a stop on the outskirts of Hyderabad, 160km north of Karachi, Pakistan's commercial capital.

The blast destroyed around a dozen vehicles and at least 10 shops.

Nawaz Abbasi, a medical superintendent at the city's Liaqat University Hospital, said his hospital had received 14 bodies, and at least 30 people wounded.

Fayyaz Leghari, a senior police officer, told the Reuters news agency no evidence suggesting it was a bomb or a suicide attack was found.

Pakistan has been hit hard by sporadic violence in recent months as it fights Taliban and al-Qaeda fighters in the northwest.

In the latest clash of its kind, security forces in the northwestern Orakzai region killed 12 fighters after their checkpoint came under attack, a government official said.

Two soldiers were wounded in the gun battle.

No independent verification of official figures of casualties was available.