TEHRAN, Iran (AP) — An explosion on Friday killed one worker and injured three others on a decades old land-based oil rig, semiofficial Mehr news agency reported.

The report said the blast erupted at the Bibi Hakimeh oil rig, about 500 miles (800 kilometers) south of the capital Tehran, starting a fire, after workers reached a significant layer of trapped gas in the field.

The field has been in operation since 1962, producing 120,000 barrels of crude a day. The report said firefighters brought the blaze under control.

Increasing incidents of explosions have hit Iran's oil and gas sector recently, but authorities rarely provide explanations for them.

The news agency said another fire Friday at Iran's Arak refinery some 180 miles (300 kilometers) southwest of Tehran set off an explosion. There were no reports of casualties.

Majid Rajabi, head of the refinery, told Iran's state radio that there was no fire or explosion in the operational part of the refinery. He said the fire might have been linked to contractors' work on development projects.

Iran is OPEC's second largest oil exporter, and sits atop 137 billion barrels of conventional crude oil. It also holds the world's second largest natural gas reserves — roughly 28 trillion cubic meters.

Some 80 percent of the country's foreign revenue comes from exporting oil.

Most of Iran's pipelines are decades old and suffer from lack of maintenance and frequent technical failures. Also, there have been occasional cases of sabotage, mostly in northwest Iran, near the Turkish border.

In August, an explosion hit an oil pipeline in Iran's oil-rich southwestern province of Khuzestan, a week after an explosion struck a major pipeline carrying gas to Turkey.

That blast, which temporarily cut the gas flow, took place in morning hours near a border crossing. No one was injured. Authorities blamed it on Kurdish rebels operating in the area.

In April, three explosions hit gas pipelines near the holy city of Qom in central Iran, briefly cutting the flow from Iran's gas refineries in the south to the country's northwest.

Similar explosions rocked the same pipelines in February. Officials at the time said the blasts were not caused by technical failures but did not say if they were acts of sabotage.