The Fars news agency did not say when the incident happened or give a source for the report on Tuesday, which comes at a time of tension between Tehran and Washington over Iran's nuclear programme.

But the US defence department said it was unaware of any US military aircraft being forced to land in Iran and that all US aircraft in the Gulf region were accounted for.

Lieutenant-Colonel Patrick Ryder said: "According to the combined air operations centre, all our aircraft are accounted for and we have no reports of any aircraft landing in Iran."

Lieutenant Stephanie  Murdock, a spokeswoman for the US Fifth Fleet, told the AFP news agency from its base in Bahrain she had "no information right now" about the Iranian reports.

Fars reported that the jet was carrying five military officials and three civilians from Turkey to Afghanistan when it "unintentionally" entered Iranian airspace.

Iranian fighters guided the jet to an Iranian airport, the passengers were questioned and a day later were released and allowed to continue to their destination, Fars said.

It said the jet was a Falcon, apparently referring to a passenger aircraft manufactured by the French firm Dassault.

'Lost its way'

Al-Alam, Iran's official Arabic-language television station, quoted an unnamed military official as saying that the jet belonged to either a British or Hungarian relief agency.

He said the aircraft "lost its way" and violated Iranian air space. He said the passengers included American military personnel but did not elaborate.

"This plane was not a military plane and did not belong to the United States," the official was quoted as saying by Al-Alam.

The aircraft was forced to land in Iran on Sunday and allowed to fly to Afghanistan on Monday, the website said.

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