CIA analysts forced 30 flights to be cancelled and raised the US terror alert from yellow to orange because they thought that al-Qaida was sending hidden messages through the headlines of the Arabic television news channel al-Jazeera, it has been revealed

According to a report by NBC, CIA experts thought they had decoded messages that they believed gave dates, flight numbers and geographic coordinates for targets that included the White House, Seattle's Space Needle and even the small town of Tappahannock, Virginia, which has a population of 2,000.

"These credible sources suggest the possibility of attacks against the homeland around the holiday season and beyond," said the homeland security chief, Tom Ridge, at the time of the incident in December 2003.

But in an interview with NBC 18 months later he conceded that the intelligence analysis was "bizarre, unique, unorthodox, unprecedented", and that "speaking for myself I've got to admit to wondering whether or not it was credible.

"Maybe that's very much the reason that you'd be worried about it, because you hadn't seen it before."

Code yellow denotes a significant risk of terror attacks. Code orange denotes a high risk, and additional precautions are taken at public events.

The analysis led to several international flights, operated by Air France, British Airways, Continental Airlines and AeroMexico, being cancelled.

Seven men - one French, one American and five Algerians - were questioned in Paris and released.

"The people with Arab-sounding names turned out to be, for example, a diplomat and a sports player. There were no terrorists," a police source told the newspaper Le Parisien at the time.

Al-Jazeera said the NBC report "vindicated" the network after repeated claims from the Bush administration and its allies that the broadcaster was linked to or sympathetic to terrorists. "We've always said these are politically motivated allegations."

The analysts were using a system called steganography, which examines hidden messages, in this case in video images.

Professor Nasir Memon of the Polytechnic University in Brooklyn, said that such an analysis is not always reliable. "It's not something I would bet the farm on because there is a significant chance that it could be wrong."

More than 20 al-Jazeera journalists have been arrested and jailed by US forces in Iraq and one was killed in April 2003 after a US tank fired a shell at the channel's Baghdad offices.