"The first phase of the air force drills started today," and the main phase beginning on September 13, deputy air force commander Brigadier General Mohsen Darehbaghi said, quoted by Fars news agency.

The deputy commander in charge of the operations, Brigadier General Mohammad Alavi, separately described the drills as "offensive," although he did not elaborate.

"In these manoeuvres, we have designed a real war zone for the pilots," as opposed to previous aerial "competitions," Alavi said.

Iran's armed forces regularly conduct such exercises to show off the country's military prowess and test-fire what they boast are home-made missiles.

"In this offensive drill we will show the world that the air force is always ready to defend the country's space," the state television website quoted Alavi as saying.

The air drills, which are to be staged over four phases, were "unprecedented since they will showcase numerous fighter sorties and tactical missions," Fars quoted Alavi as saying.

Tehran is under mounting international pressure over its controversial nuclear programme, which the West fears masks a drive to acquire atomic weapons capability -- a charge the Islamic republic persistently denies.

The United States and Israel have not ruled out a military strike to curb Iran's nuclear drive, prompting Tehran to threaten a crushing response.

"The secret service has opened an investigation," its spokesman Vincent van Steen told AFP, adding "it is specifically to find out who hacked the certificates."

The ministry said Monday that Internet security specialist company Fox-IT released a report noting that hackers in July falsified 531 Internet security certificates, also known as SSLs, through Dutch company DigiNotar.

The hackers tried to intercept private communications in Iran, Fox-IT said.

SSL certificates are used to verify to visitors that a particular website is authentic and are issued by DigiNotar and other firms known as Certification Authorities. Internet users whose browsers are fooled by a false certificate could unwittingly reveal their activity to another party in what is known as a "man-in-the-middle attack."

According to Fox-IT, a number of the DigiNotar's servers were hacked and false certificates issued to websites not only that of Skype, Google, Twitter and Facebook, but also websites belonging to spy agencies the Central Intelligence Agency and and Israel's Mossad.

"It wouldn't surprise me in the end if it turns out that the Iranian government is behind it and is trying to get information it normally couldn't have, such as that from dissidents," Hans van de Looy, an expert at computer security specialist Madison Gurkha, told AFP.

Fox-IT said 99 percent of users targeted by cyber attacks were in Iran.