It was not clear whether the plane was Iranian or foreign, although the influential Kayhan newspaper said that "usually these sort of planes are used for spying on other countries".

The reports quoted Ali Asgar Ahmadi, deputy head of security in the Interior Ministry, as saying the plane went down on Thursday in the Alashtar mountains near the city of Khorramabad, the capital of Lorestan province, 350km southwest of Tehran.

Spying instrument?

Kayhan said that as soon as the plane crashed, police sealed off the area - just 150km from the border with Iraq - and "a group of experts from Kermanshahr airbase went to examine the fuselage".

"It is under investigation," a local official was quoted as saying.

No further details were given.

Earlier this year the former intelligence minister Ali Yunessi confirmed the presence of "American spying instruments" in the skies over Iran and warned that they would be targeted by the military.

"Americans have been conducting spying activities in the Iranian sky for a long time," he said in February.

US media reports this year also said the United States has been flying drones over Iran since April 2004, seeking evidence to back up its claims that Iran is working on nuclear weapons and probing for weaknesses in Iran's air defences.

The administration of US President George Bush has refused to rule out possible military action over Iran's nuclear activities, saying that its efforts to develop nuclear fuel are a cover for an atomic weapons programme.