Tehran: A massive explosion and fire rocked a port Saturday in southern Iran purportedly linked to a shipment of a chemical ingredient used to make missile propellant, killing five people and injuring more than 700 others.

Mehrdad Hasanzadeh, a provincial disaster management official, gave the updated casualty figure in an interview on state television. The previous toll was four dead and more than 500 others injured.

Social media videos showed black billowing smoke after the blast. Others showed glass blown out of buildings kilometers (miles) away from the epicentre of the explosion.

State media footage showed the injured crowding into at least one hospital, with ambulances arriving as medics rushed one person by on a stretcher.

Authorities have offered no cause for the explosion yet. Industrial accidents happen in Iran, particularly at its aging oil facilities that struggle for access to parts under international sanctions. But Iranian state TV specifically ruled out any energy infrastructure as causing or being damaged in the blast.

Mehrdad Hasanzadeh, a provincial disaster management official, told Iranian state TV that first responders were trying to reach the area while others were attempting to evacuate the site.

Hasanzadeh said the blast came from containers at Rajaei port in the city, without elaborating. State TV also reported there had been a building collapse caused by the explosion, though there were no immediate other details offered.

Rajaei port is some 1,050 kilometres southeast of Iran’s capital, Tehran, on the Strait of Hormuz, the narrow mouth of the Persian Gulf through which 20 per cent of all oil traded passes.

The blast happened as Iran and the United States met Saturday in Oman for the third round of negotiations over Tehran’s rapidly advancing nuclear programme.