Tehran - The Iranian government on Sunday announced that Oil Minister Kazem Vaziri Hamaneh had stepped down - the second resignation from President Mahmoud Ahmadinejad's cabinet within a day.

The surprise announcement came in a decree by Ahmadinejad quoted by the ISNA agency which said Vaziri Hamaneh had been appointed a special advisor to the president in oil and gas affairs.

His replacement as oil minister of Iran - Opec's number two producer and world's fourth biggest - will be Gholam Hossein Nozari, head of the National Iranian Oil Company (NIOC), according to the decree.

"You are appointed acting head of the oil ministry," said a decree addressed to Nozari. "I hope that with the co-operation of the cabinet you will fulfill your duties in full compliance with the main programmes of the government."

Vaziri Hamaneh meanwhile was thanked "for your sincere work" as oil minister.

The reasons for the change were not immediately clear.

The switch came on the same day that Iranian Industry Minister Ali Reza Tahmasebi resigned in what the government said was a move aimed at increasing co-ordination with the industrial sector.

Mojtaba Samareh Hashemi, a top advisor to Ahmadinejad, played down the changes, saying the cabinet remained cohesive and unified.

"This does not mean that those who have resigned are exempted from responsibility and they will not cooperate with the government team but these persons are given new responsibilities," he said.

Vaziri Hamaneh was Ahmadinejad's fourth choice for oil minister when he became president in August 2005 after previous candidates were vetoed by the conservative parliament.

He oversaw lucrative receipts from oil exports due to the high price of crude and also supervised the implementation of the government's petrol rationing drive.

But Iran's oil sector - vital for the wider economy and responsible for 80 percent of its export earnings - has been suffering from a lack of investment over past years.

The problem has been exacerbated by the unwillingness of European banks to lend money to Iran in the face of US pressure linked to its controversial nuclear programme.

Kamal Daneshyar, the head of the parliament's energy commission, said that the two ministers had been in disagreement with certain methods of the government.

"It seems that certain methods do not suit the thinking and the action of the two ministers, and it is for this reason that they were replaced," he added.

Vaziri Hamaneh becomes the fourth minister to step down since Ahmadinejad became president.

The original social affairs and co-operatives ministers have both resigned. The education and agriculture ministers have each survived impeachment votes in parliament.

The resignations of the two ministers come at a time when Iran's economy is experiencing problems with double-digit inflation, poverty and high unemployment.

Ahmadinejad was elected on a platform letting all of Iran's population feel the benefits of its oil wealth but poverty remains a major problem in the Islamic republic with over 13 percent living beneath the poverty line.

Samareh Hashemi added that in the coming days Ahmadinejad would appoint Ali Akbar Mehrabian as acting head of the industry ministry. Mehrabian is currently Ahmadinejad's special representative on the fuel consumption committee.