Listening to his radio, this ham operator from the Netherlands was able to monitor radio exchanges between civilian and military flights in the region and make his findings public on his Twitter account @FMCNL.

Hunched over his radio, he listens in on unencrypted exchanges among military aircraft and their air traffic control centres.

The messages contain information about the location of the planes, which is necessary to avoid collisions between military and civilian aircraft.

Yesterday the radio operator posted the following message on Twitter: "PSYOPS is running! USAF EC-130J tail nr 00-1934 callsign STEEL 74 transmitting messages to Libya on HF freq OdysseeDawn."

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The cryptic text refers to an EC-130J aircraft, a modified version of the US Air Force's Hercules plane, whose registration number is 00-1934 and whose code name is "Steel 74".

The aircraft beams high-frequency messages on Libya as part of Operation "Odyssey Dawn," the international military campaign against forces of Libyan leader Muammar Gaddafi.

The plane, belonging to US special forces, is used for propaganda and psychological warfare.

It flies at high altitudes and broadcasts messages to influence the behaviour of enemy forces or the population at large.

Asked at a press conference about these operations, a senior Pentagon official confirmed that a "specialised aircraft" had been used by coalition forces.

"We've put up one of our specialised aircraft of that nature and I'm not ready, I'm not able to to talk about the messages," Vice Admiral Bill Gortney told reporters.

But one of the messages broadcast by the plane has been recorded by the Dutch ham radio operator and posted on the internet.

Speaking in English, French and Arabic over background noise, a man tries to deter Libyan vessels from going out to sea after coalition navy ships established a naval blockade.

"Libyan ships or vessels do not leave port," the message says.

"The Gaddafi regime forces are violating a United Nations resolution ordering the end to the hostilities in your country. If you attempt to leave port, you will be attacked and destroyed immediately. For your own safety do not leave port."

According to the Danger Room blog, which specialises in security issues, the amateur radio operator who made this discovery is a former Dutch military man known as "Huub".

His Twitter postings are monitored by more than 5000 subscribers.