According to the Aftab newspaper, the latest violation came on Saturday when a US fighter flew at low altitude over an area in the northeastern province of Khorrasan which borders Afghanistan.

Other press reports said the overflight followed a recent intrusion by F-16 and F-18 fighters over the southwestern province of Khuzestan which borders southern Iraq. Papers said the planes appeared to be spying on nuclear sites.

No further details or official confirmation were immediately available.

Last month the Iranian army's chief General Mohammad Salimi said the Iranian military led by the air force has been ordered to stand ready to defend the country's nuclear sites in case of attack.

Israel and the United States accuse Iran of seeking to develop nuclear weapons, leading to speculation over the possibility of military strikes. Iran insists its nuclear programme is strictly peaceful.

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