An accidental blast at Iran's Abadan's oil refinery killed at least one person and injured more than 20 during a visit Tuesday by President Mahmoud Ahmadinejad, who was not affected and went on to make a televised speech, reports said.

"A technical fault led to an explosion in one of the units of this refinery," Fars news agency said, referring to the oil facility in the southwestern city of Abadan.

"The fire was immediately controlled ... The blast did not interrupt the president's programme," Fars added, saying heavy smoke had clouded the refinery and billowed across the city.

Mehr news agency initially said one person was killed in the accident and another 10 injured but in a later report said 22 people had been "affected" by the blast, without elaborating on the number of dead or injured.

"Currently the control of the fire and the rescue operation has been completed and the investigation of the blast has commenced," Mehr said.

"The leak of gas in one of the refinery's units has been announced and is the main cause of the blast and the fire," it added.

"The spread of poisonous gas has resulted in gas inhalation and intoxication of some of the employees."

Ahmadinejad was at the refinery to inaugurate a unit that will elevate the nation's petrol production capacity by an extra 4.2 million litres a day.

Iran currently produces 54 million litres of petrol per day -- below the consumption level of around 50 million litres a day.

Several accidental explosions were reported last year in Iranian oil and gas infrastructure, resulting in the sacking of some local officials accused of negligence.

Last September, up to 10 people were killed in a gas pipeline explosion near Iran's northeastern city of Mashhad. Sixteen others sustained burns.

The main pipeline bringing gas from southern Iran to Tehran has been sabotaged twice this year already, according to Iranian media reports.

The acts of sabotage, which have neither have been claimed nor attributed by authorities to any specific groups, did not cause any casualties nor disruption to supplies to the capital.

Iran, the second largest exporter of crude in the Organisation of Petroleum Exporting Countries, holds the world's second largest gas reserves.