DUBAI - Iran was investigating a suspected cyber attack on its main oil export terminal and on the Oil Ministry itself, Iranian industry sources said on Monday. 

A virus was detected inside the control systems of Kharg Island, the country's largest crude oil export facility, but the terminal remained operational, a source at the National Iranian Oil Company (NIOC) told Reuters. 

Officials said the attack had not corrupted vital information at NIOC, although it had damaged general information, an oil ministry official told the semi-official Fars news agency, which has some ties to the government. 

"This cyber attack has not damaged the main data of the oil ministry and the National Iranian Oil Company (NIOC) since the general servers are separate from the main servers, even their cables are not linked to each other and are not linked to internet service," Alireza Nikzad told the agency.

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The virus, which is likely to be compared to the Stuxnet computer worm which reportedly affected Iranian nuclear facilities in 2009-10, struck late on Sunday, Reuters reported. 

It hit the internet and communications systems of Iran's Oil Ministry and of its national oil company, the Mehr news agency -- which calls itself "private and non-official media"-- reported. Computer systems controlling a number of Iran's other oil facilities have been disconnected from the Internet as a precaution, the agency added. 

Hamdullah Mohammadnejad, the head of civil defense at the oil ministry, was reported as saying Iranian authorities had set up a crisis unit and were working out how to neutralize the attacks. 

IT systems at the oil ministry and at the national oil company were also disconnected to prevent the spread of any virus, the Mehr news agency said. 

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The oil ministry's own media network, Shana, quoted a spokesman as saying no major damage had been sustained. 

Iran's nuclear program is thought to be the principal target of the Stuxnet worm -- discovered in 2010 -- the first virus believed to have been specifically designed to subvert industrial systems. 

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U.S.-based think-tank, the Institute for Science and International Security (ISIS), said that in late 2009 or early 2010 about 1,000 centrifuges -- machines used to refine uranium - out of the 9,000 used at Iran's Natanz enrichment plant, had been knocked out by the virus -- not enough to seriously harm its operations. 

Iran also identified damage inflicted by a similar virus aimed at disrupting industrial processes, called Duqu. Experts say Duqu appears to be designed to gather data to make it easier to launch future attacks and that very few organizations could have written such complex programs. 

Most of the world's oil facilities are controlled by computers, but some processes can be controlled manually when necessary. A shipping source with knowledge of operations at Kharg Island said that NIOC has been prevented from sending out the crude loading program at the terminal.   

Reuters contributed to this report. 

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TEHRAN, Iran -- A senior Iranian military official has claimed the country's nuclear facilities are immune to cyber attack.

Gholam Reza Jalali, who heads an Iranian military unit in charge of combating sabotage, was quoted Monday by the official IRNA news agency as saying that Iran and its nuclear facilities possess the technology and knowledge to deal with malicious software, according to The Associated Press.

He did not specify what steps have been taken since 2010, when a virus known as Stuxnet disrupted controls of some nuclear centrifuges. Tehran says its scientists neutralized the malware before it caused serious damage.

Iran has reported other cyber attacks since, including an infection in April 2011 dubbed "Stars." Jalali said that Iran also fought a spy virus called "Doku," without providing details.

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Iran's claim comes amid rising tensions between the Islamic Republic and the West.  As a tightening web of international measures aims at forcing the Islamic Republic to scrap sensitive nuclear work, a string of events have made it look to the outside world like an undeclared "soft war" was under way.

Apart from the Stuxnet computer virus that attacked centrifuges at the Natanz enrichment facility, which Iran accused Israel and the United States for, the U.S. has accused Tehran's shadowy Quds Force in a plot to assassinate the Saudi ambassador to Washington.

In late 2011, the United States lost a spy drone in Iran, unmasking an aggressive surveillance program.

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There have also been unexplained explosions at an Iranian missile depot and four nuclear scientists have been killed in Iran-- the latest on Wednesday.

Iran reacted to those events and the stepped-up economic sanctions as if under siege. It threatened to close the Strait of Hormuz, a major oil shipping lane.

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The Associated Press, Reuters and msnbc.com staff contributed to this report.