Iranian President Mahmoud Ahmadinejad announced the dismantling of four key government ministries. [AFP]

Three Iranian ministers have been dismissed and their departments dismantled in a government restructuring that cuts the number of ministries from 21 to 17, Mahmoud Ahmadinejad, Iran's president, has announced.

Ahmadinejad announced the sackings of the country's ministers of oil, social welfare, and industry and mining, following a recent law passed by parliament which requires him to shrink the country's number of ministries, the ISNA news agency reported on Saturday.

While a change was expected, the sudden dismissal comes as a surprise, as all three were considered staunch supporters of Ahmedinejad, and because Iran's parliament is still discussing an earlier proposal suggesting that the country's oil and energy ministries, as well as labour and social welfare ministries, should be merged.

It was not immediately clear who would take over the merged agencies. According to Iranian law, parliament has to approve the remit of any new ministry.

But parliamentary approval could be a difficult path, as parliamentary speaker Ali Larijani has been publicly critical of Ahmadinejad's domestic and economic policies.

For the world's fifth largest oil exporter, the abolition of the oil ministry could have major effects, although the specific impact remains unclear.