An explosion blamed on a gas leak has rocked Iran's largest oil refinery shortly before the arrival of President Mahmoud Ahmadinejad for a scheduled visit, an Iranian news agency has reported. The semi-official Fars news agency said the blast occurred on Tuesday morning, just before Ahmadinejad was to inaugurate an expansion project at the refinery in the south-western city of Abadan. At least 20 people were injured in the blast, which set off a fire at the site, the report said. The explosion was blamed on a "gas leakage" but there were no further details. Another report, by the semi-official Mehr news agency, said at least two people were killed and that the explosion happened while Ahmadinejad was visiting. It said the president ordered a special plane to airlift the critically injured to Tehran.